Origonal MP5's
It is a little know fact and often gotten wrong, that there are no transferable 'factory' MP5's in the registry. All available MP5's that are out there are in fact conversions. H&K only made their MP5's for Military Units, but a few "Dealer Samples" made it into circulation. The 'factory' MP5's that you are referencing are the simi-autos, the 94's and SP-89's that H&K took off their simi line and ran through the MP5 line basically. As they are genuine H&K converted full auto's they did not begin their life as an MP5. (one of my local gun dealers used to be a saleman for H&K.. He could only sell the MP5 to Military Units in the US. He could sell the factory conversions to Police and dealers.
Several high end conversions like those of T. Dyer, John Flemming, Qualified, and Ralf Smith (RDTS) (several others also did conversions.. I just cant remember the names... I think Vector did them as well, but I dont remember for sure) were converted to use push pins and had the receiver itself registered as the gun where-as they also registered the sear and the sear/trigger pack housing as the machine gun as well...
This is how you get "Registered Receiver" (Where the receiver is registered), "Registered Sear" where the "H" style sear of the weapon is registered, and then the "Sear Pack or Registered Pack" (the houseing that holds the trigger group/FCG) is registered.
The 3 pin, swing downs are the most desirable as they are true EXACT copies of the factory MP5 (if you're a purist), then the next is the Sear Pack as they slip in and out of H&K style weapons with very little to no gun smithing required (addition of full auto bolt/carrier which most clones already have), and then the sear where a qualified H&K knowlegable smith is required to install it (to make sure the timing of the SEAR/hammer strike is properly tuned).
If you own a SEAR or a PACK, purcase a clone registered as a 'pistol'. Once you add the sear you can attach a stock and VFG and anything else you want to the weapon. So its now a 1 stamp gun. If you ever take out the sear or pack you have to remove the FVG and the stock...