After it is made.

As stated on the form:

Can't say I'm an expert on the Mfg. of suppressors and the ins and outs there.  My logical assumption would be when the assembly of the can is complete and it reaches the point of being functional and gets the serial number engraved.

Someone like @HansohnBrothers or @ECCO_Machine could say for sure.

Generally an 07/02 can make unserialized but restricted suppressor components for inventory pursuant to manufacturing complete and registered suppressors.  

I suspect a lot of what is allowed  comes down to some level of common sense and what your IOI considers reasonable for your business.  Say you have never manufactured a suppressor or registered one in the NFRTR after being in business for 5 years and you work out of your barn behind your home. Yet for some unexplainable reason during an inspection you have 1000 cut, threaded, and unserialized tubes and 5000 machined baffles that fit those tubes in your inventory.  That might cause some level of concern during an inspection unless you can articulate why you have all these restricted suppressor parts with no viable plan to turn them into registered suppressors.  

However, if you are Silencerco and have a 1000 cut tubes and 5000 baffles in your inventory because you make 500 suppressors a day and you have a 10,000 unit backorder,  it might be completely customary for your business to have a couple days of raw component inventory and it raises no concerns.

In terms of outsourcing manufacturing work you generally need to get a variance letter from the ATF if a serialized part is going offsite for work and the other FFL doesn't want to mark the receiver with their manufacturer information.   Other parts like sending out flash hiders for coating might not require any approval or the sub to be an FFL since a flash hider is not a restricted part like a receiver is.   For things like NFA firearm receiver manufacturing where another shop is doing a manufacturer activity on your receiver (welding, machining, etc.) you probably either need some sort of variance letter and/or an NFA transfer being done since you generally can't transfer NFA firearms without approval and that sub is probably almost always going to need to be a 07/02 as well.  If you sub out things like suppressor baffles where the parts themselves are restricted but also unserialized I am not sure what level of approval may be required to have a sub-contractor make restricted baffles for you but I suspect they are going to need to be an SOT manufacturer as well.

This is exactly what got Anzio Iron in the wringing with the BATFE.  The ATF would come in and do inspections and see complete guns but no onsite manufacturing capability of  producing them.   After investigating they found out that Anzio was illegally subbing manufacturing to another non-licensed site and after getting caught once, did it again.

https://casetext.com/case/anzio-ironworks-corp-v-gerber

I don't disagree with you and I presume like a lot of thing with the BATFE it comes down to their interpretation on how things went and whether they choose to make your life a nightmare as a result.

Technically sure if an FFL/SOT wants to buy maglights from home depot, harvest the tube bodies, make baffles and endcaps that fit them, and register it as suppressor its probably more difficult for the BATFE to come after you given how common and multi-use a maglight is.  if you choose to use maglight bodies as your tube raw material source because they are cheap, come in a variety of colors and sizes,  and are available at the corner store on a moments notice that might make sense.

Now lets say in this hypothetical,  the same FFL contacts the local speed shop and says....hey, bob....can you make some "inter-cooler connector tubes" out of this specific grade titanium, exactly 11.5" long, threaded to this specific spec and then let me buy them sell them to me as unrestricted inter-cooler tubes.  I then register and sell them and we all make money.

One day and the BATFE comes in to so an inspection and  finds a stack of these threaded titanium tubes and no clear in house capability to make them and asks where you got them from.  The ATF then goes and squeezes the speed-shops guys nuts until he coughs up that you gave him the tube specs and that you effectively colluded with the speedshop to do offsite NFA manufacturing with a non-FFL/SOT under the guise these were inter-cooler tubes.  The BATFE probably come back to you and say,  Bob spilled the beans on your silencer manufacturing collusion scheme and gave us the text messages proving it.   You can surrender your license or we can throw all these criminal charges at you...at which point you can go out of business or cough up a $50K attorney retainer to get started.

Obviously there is a lot of gray somewhere in between these two scenarios.  However, given the BATFEs seemingly almost endless ability to regulate FFLs and manufacturer activity and to make FFLs lives difficult if you do something they don't feel is above board, it seems like the best option is to genuinely make everything yourself from true raw stock or get approval to sub-contract restricted parts before you do so.

There is probably also some level of selective enforcement where say FN or Colt the BATFE doesn't really hassle them when they bring on a new sub-contractor for parts or work vs. a sole prop barn based FFL/SOT who is buying "80%" drop in auto sears, solvent traps kits or oil filter adapters to build regulated NFA firearms with.