Hello fellow retro enthusiasts:

Question 1:  Is the Colt 604 the only variant that was marked M16 on the magwell, between the US Govt property marking and Caliber?  I've looked around a lot online, and in multiple reference books, and the only rifle variant I've seen that is marked M16 (not M16A1, or XM16, or anything other than M16) is the Colt 604.

Question 2:  The Colt's Firearms Division markings came in the mid 1970s, correct?  Is there any way a 604 would be marked with Colt's Firearms Division?  Every 604 I've seen pictures of is marked Colt's Patent Firearms Mfg.

The reason I ask is that I bought an engraved 80% a few years ago, and at the time I wasn't that obsessed with correct markings.  Now that I'm neck deep in the retro obsession, the markings on this lower aren't working for me.  I'll probably just use it for a truck gun, but if there's any way a 604 would be marked with Colt's Firearms Division, I'll go through the trouble of converting this lower to an A1 and build a 604 clone with it.

Where to start... It appears to have a black finish and its an A2 lower.

With an A2 grip

The Colt company name officially changed to Colt's Firearms Division sometime in 1964, but there were tens or hundred of thousands of receivers in inventory with the "Patent Firearms" markings, so the newer markings didn't become broadly seen until the later 1960s, or even the 1970s where the SP1 receivers were concerned.

Markings are fine; color and A2 reinforcements are not. Just get those corrected.

Eh, no, markings are not so fine. You wouldn't have a Colt Industries marked lower in the 100000 serial number range. That's in very early 604 territory, like concurrent with the 602 and very early XM16. Something odd about the layout as well... doesn't seem to fill the area like it should. It's just a mishmash of Colt rollmarks thrown together to be "acceptable" to the untrained eye. And it's on an A2 lower...

There were 604’s made in 70 which is when the rollmark on the back half of the lower changed.

The serial number is wrong for it to be marked Colt Firearms Division and it’s the wrong “Hartford Horsie” for that serial number as well, if the SN was in the 4.8M range it would be correct.  Also as others have mentioned I’d be more annoyed that it’s on a black A2 profile lower

I’d say get it fixed to look like an a1 lower, and add a 4 to the serial number

There’s even the correct upper for this time frame on gunbroker currently