Wow, you are lucky...you don't see these suits too often anymore. They were being phased out when I was on active duty and the POTMC suit's purpose has been filled with various Level A suits (another encapsuilation suit.) These were used for EOD missions so I didn't see them during my career (I was never EOD qualified). I did some googling and came up with some info though:
Hi, I’m in the process of buying a M50 respirator from in the military and was wondering if there is any paperwork I should ask for to prove that it’s not stolen or anything and you seem like someone who would be able to provide the answer I’m looking for.
Thanks in advance!
You will want to keep the recipt of course or at least a photo of it to archive. However, I wouldn't be too concerned. There are hundreds of thousands in the system and many will find their way to the secondary market. Most of the time, a mask is issued and not retrieved by a supply sergeant after the Soldier PCSs or ETSs. These are sold to the local pawn shop or military surplus store. The military itself might sell these at DRMO for servicability reasons or simply because of oversupply in some region. If you are too concerned, Avon will gladly sell you a C50 mask which is the same mask but for the civilian market. However, I don't think anyone will challenge you owning an M50 protective mask.