First:
The FFL (not ffa) you allude to is strictly for selling any gun for a business or manufacturer. Don’t need it to own a FA, nor are all FFLs what they call Class 3 FFLs, most aren’t. You yourself can buy one, just pay the 200 dollar tax stamp to the ATF and wait about a year for the background check to go through.
As to the M203, that can be found. But it’s a 5 dollar federal tax stamp and registration as an other than destructive Device.
The reason you can’t legall buy the part and then register a new FA is because the Hughes amendment of the 1968 gun control act put a closing date of May 1986 on registerations. Every FA had to be registered by that date. Anything not, became illegal.
Now there is a manufacturer exception to that IF you have the class 6 I believe it is, FFL. But to get that, it’s some 5000 bucks a year plus you have to have a minimum sales requirement.