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IMHO, the item in the auction falls in the same questionable category as the married sear guns (i.e. The sear is the MG and should have been registered, but a semi auto firearm was registered instead. In an M2 conversion kit, the registered part is supposed to be one of the parts in the ATF specified conversion kit. The trigger housing is not one of those parts...
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That is an interesting question.
The seven restricted M2 parts are the hammer, selector lever, selector lever spring, disco lever, disconnector, disconnector spring, ad disconnector plunger.
These parts have been classified by the ATF as having no other purpose than to convert an semi-automatic M1 into a machinegun.
Therefore possession of some or all of these 7 parts with a semi-auto M1 and/or having these parts installed into an M1 is somewhat analogous to being in possession of an M16 hammer, trigger, disconnector, and selector along with an AR15. (or have them installed into an AR15)
Get caught with these parts in your AR15 or your M1 carbine (the more you have the worse it probably is from a legal standpoint) and it could be bad news for you if caught by the proper authorities.
That said there are some M2 specific parts that the ATF doesn't consider as specifically designed to convert a M1 into a machinegun and can legally be used in an M1. These are the M2 stock, trigger frame, sear, and slide. These parts are sort of like an M16 bolt carrier. While designed for use in the M16, the M16 bolt carrier can be used in an AR15 as it really doesn’t convert an AR15 into a machinegun.
As circuits mentioned the M1 is somewhat unique in the machinegun world in that the M1 receiver itself requires no modification to use factory designed parts to convert it mechanically into an M2 machinegun.
In this case while certain M2 specific parts are specifically considered machinegun conversion parts unto themselves…the question is….. does that necessarily preclude you from registering another non-restricted M2 part (like the trigger frame) as the machinegun conversion device when combined with all the other listed parts above?
I don’t recall ever seeing an official ruling stating that you had to register one of the 7 initial parts listed above and you couldn’t register the trigger frame itself which holds those 7 parts as long as all those parts were together as a conversion kit at the time of registration. (for practical purposes only the selector, disco, hammer, and disco lever could be engraved anyway). I have also seen registered M2 slides as well (which in my estimation is a bit more of a stretch from a legal perspective)
I could see it similar to an HK or AUG conversion trigger packs that use a conversion style sear but the actual NFA engraving is on the fire control box or hammer pack body itself. (I think that lafrance did some like this)
From a modern day standpoint, assuming you were converting a PS90 to a machinegun via a modification to the semi hammer pack. This can be done with no mods to the hammer pack itself and with just a small piece of bent wire. I would think you would engrave the hammer pack body itself (even though its not technically a restricted part) as it is now one of a combination of parts intended specifically to convert a PS90 to machinegun and the most convenient part to engrave.
There are certainly a lot of M2 fire-control conversion kits out there and some of them have the NFA markings on the trigger frame itself making for easier identification once installed.
As to the OPs question about usage. This pack would drop into any virtually any type of M1 carbine and all you would have to do was make sure the stock had the proper relief cutouts and the correct M2 slide was used. Think of this like M2 conversion kit to a DLO HK box but that still requires a full auto carrier to work.