Why is this (conversion) type of sear/catch any different than a factory replacement sear/catch.....which has no restrictions on possession? Because this type of sear was created specifically to convert a semi auto to full auto......utilizing a different geometry than the factory sear/catch. The BATF views conversion sears as machine guns and those conversion sears have to either: be registered (like Fleming, Qualified or S&H sears), or permanently married to its registered "spouse" (LaFrance pack or Clip-on RR).
Separating (permanently) an unregistered conversion sear from its "spouse" creates a 2nd machine gun in the eyes of the BATF and is very much afoul of current regs.....even if the original conversion sear is destroyed when performing pack work. Is there a high probability if a LaFrance registered pack (or Clip-on RR) which had trigger pack work performed and the conversion sear was tossed/destroyed.......that such would be discovered by BATF? Probably not.....and if it were discovered by BATF, you probably have much bigger legal problems which led them to be "into your stuff" that deep in the first place. Regardless, one never knows.
My personal concern would be the impact on the VALUE of owning and at some point trying to sell something with questionable legal status. Were I in the market for a LaFrance pack or Clip-RR for that matter, especially if it was in the form of / came with an Ambi pack lower.......would be to confirm the pack still contained that original conversion sear. If it did not.....I personally would not touch it and I think most of the viable market wouldn't either at the price level these items are commanding.
This scenario is no different than converting a RR Uzi with slotted bolt to a "full machine gun conversion" and leaving an orphaned slotted conversion bolt -or- converting a Clip-on RR HK to a Push Pin RR and leaving an orphaned conversion sear. Both are expressly forbidden by BATF in writing....even if the conversion devices are destroyed.
If a LaFrance pack can, and is converted to an 4-position ambi pack with it's proprietary conversion sear in place......no problem. If the conversion results in the proprietary conversion sear being orphaned.......then I would say someone has a legal conundrum on their hands and has significantly harmed its future value to anyone who is the least bit informed on the matter.
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However, I would agree you probably have to keep the conversion sear in the mix. Setting up the pack to take a factory/unregulated HK sear, orphans an unregistered machinegun conversion component.
Odds of being caught are probably slim but agree that converting a LaFrance pack to Ambi/Burst could quite possible damage its value, even if you kept the custom sear in the mix as folks are going to know a lot of work (i.e. welding) had to be done to move that sear axle pin location. If the LaFrance conversion sear is removed/destroyed during the Ambi/Burst conversion process, after seeing those pictures I probably wouldn't touch a LaFrance pack that uses a factory german sear. Why take the risk when DLO and registered sears are out there.
At a minimum I would get it in writing from the ATF surround the proposed conversion process. However, if you really want an Ambi/Burst pack I would just get a registered sear or a DLO box and enjoy the LaFrance packs the way they are as SEF. (Two out of three of my sears are set up as SEF)