I am unable to find the requirement for putting a serial number on a finished 80% lower. It is my understanding no serial # is required by the ATF, only "strongly" suggested for ID purposes in case of loss or theft. Any help would be appreciated.

You can mark ATF down as the serial number.  

Seriously, no markings are required unless you Form 1 it later on.

.....or you transfer it (sell or give) to someone else.

Unless you have documentation to the contrary, that does not appear to be the case.

You don't have to put a number on 80% lowers, even if you decide to sell or give away at a later date.

From what I'm hearing CA has state serial number requirement that kicks in January 2018

I'm sorry if I posted erroneous information. I know that I've read that on multiple occasions.
But if there is no documentation from the ATF, then it is false and I concede.
I seem to always default back to Illinios State Law and forget that every state has different firearm laws.
Under certain conditions, person to person transfers in Illinois DO REQUIRE a record of the serial number, if available.
I always forget that Illinois has some of the strictest gun laws of all the states. FOID cards, no suppressors, etc...

No need for a serial number ... move on and enjoy your weapon.

_{You are not alone in it.  I know it has been repeated a lot in tech branch letters but can't find it in the law.}

One example:
This refers to your letter of July 27, 2004, to the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), Firearms Technology Branch (FTB), in which you posed several questions. In general, you requested clarification regarding the classification of Hungarian AMD-63 and AMD-65 selective-fire rifle parts sets, as well as information regarding the manufacture of semiautomatic copies of these firearms.

As you know, the National Firearms Act (NFA), 26 U.S.C. § 5845(b), defines the term “machinegun” as follows:

…any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger. This term shall also include the frame or receiver of any such weapon, any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun, and any combination of parts from which a machinegun can be assembled if such parts are in the possession or under the control of a person.

Further, with respect to your inquiry regarding the classification of the forward grip of the AMD-63 or AMD-65 based on 27 CFR § 478.39 (formerly 178.39), you should be aware that this is an integral part of the forearm, and, consequently would serve as the “forearm” as listed in 478.39(c)(1 7).

Concerning the manufacture of a semiautomatic copy of the AMD-63 or AMD-65 selective-fire rifle, this is possible only if the receiver is redesigned and incapable of accommodating the original fire control components. Further, the firearm must be designed to operate from the closed-bolt position. The redesigned receiver may be manufactured from new material or from the remnants of properly destroyed receivers. Proper destruction entails the diagonal torch cutting of the receiver in three critical locations. Each cut must displace at least ¼ inch of material.

We should add that prior to utilizing sections of a destroyed receiver, the features that make it capable of firing automatically—the machinegun sear pin hole and the slotted right receiver rail must be removed.

Also, for your information, a nonlicensee may manufacture a semiautomatic rifle for his or her own personal use. As long as the firearm remains in the custody of the person who manufactured it, the firearm need not be marked with a serial number or name and location of the manufacturer. However, if the firearm is transferred to another party at some point in the future, the firearm must be marked in accordance with the provisions set forth in 27 CFR § 478.92 (formerly 178.92).

Finally, a licensed collector may acquire, hold, or dispose of firearms classified as “curios or relics” as that term is defined in 27 CFR 478.11 (formerly 178.11). The collector’s license does not authorize the holder to manufacture a firearm for commercial resale.

For further details on issues related to your inquiry, please refer to the enclosed ATF publication, Federal Firearms Regulations Reference Guide 2000 (ATF P 5300.4). The text of 27 CFR 478.41(d) (formerly 178.41) appears on page 53; that of 27 CFR 478.93 (formerly 178.93), on page 58.

We thank you for your inquiry and trust that the foregoing has been responsive.

Sincerely yours,

Sterling Nixon

Thanks for that.
I did a bit more digging on the net and found reference to several states who have laws regarding serial numbers and transfers.
So the simple answer seems to be no matter what ATF has said or not said, check your state and/or local laws first.

https://www.law.cornell.edu/cfr/text/27/478.92

The actual code says nothing about markings by an unlicensed manufacturer.  And lately I am gaining an education on the utility of BATFE letters.  

I am in agreement.  It is always hard to prove something is legal, because absent a law otherwise it is.  Much more difficult to be certain that you are up to date on code, rules and court rulings and there is nothing relevant.

I always thought you were not allowed to sell a completed 80% lower with out a manufacturer license from the ATF?

That doesn't seem to be a documented restriction.  Pretty clearly, you'll get in trouble if you're repeatedly selling completed 80% lowers @ a gun show or to gang bangers.  It's clear as mud if it's legal to give away completed 80% lowers.  I ran a thread a few years back and there was no consensus.

It's mostly a question of intent.  In you finish 80% lowers with the intent to sell them, you need a licence.

If you finish an 80% lower for your own personal use, and then one day in the future decide to sell that lower, you do not need a licence.

I would not recommend selling a lower that you completed the day before, because if caught you will be ruled against and they will say you built it with intent of selling it, now if you built one today and kept and shot it for a couple of years, not showing any intent to produce for sale, then you are perfectly legal to sell it or give it away and you don't have to legally put a serial number or any markings on it.