Sorry for coming in late to the thread. I wandered away from the Forum for a while earlier this year and have not been logging in as often as I had in the past. Just happened to see you posted this thread.
I did get a Pm from Rhinohyde and sent him some info.
Yes I actually have 3 of the MK-95s, 2 50's and one 54. The MK-95 50 cal was my first inline.
The MK-95 is Not FPJ compatible when using the Posifire ignition. I suppose it could be if you had the #11 ignition installed and used the Plug for 209's (from the MK-85) but I personally don't see any need for that. It would be the same as installing it in the MK-85.
As you might have found there really are no parts left for them. I have had to have some made. The Trigger, sights, Plunger springs and some of the #11 ignition parts plug are compatible between the MK-95 and the MK-85. On the #11 plunger kit the End cap is the one part there that is different. The MK-85 end cap is Male threaded and the MK-95 is Female threaded.
A couple good things about the posifire is that the plastic case makes the ignition pretty water resistant (Not sure its completely water proof but close. The second is that since the Posifire used Large Rifle Mag Primers it does not wear the Flash holes like any of the 209s do. One good plug will last a Very Long time. I have used mine in some Very Heavy down pouring rains and never had ignition issues, and I did not go out of my way to cover the action up. Just a Kids Balloon over the muzzle.
I just looked around at places I normally got the Speer Plastic cases and could not find any either. That seems to apply to a lot of stuff at the moment.
I saw you might have gotten some of the cases. I can't remember how may I have put away but might have some.
If you want anymore info in the MK-95 Post it up and I will try to keep track.