The back end of the piston is set up that way because your Quattro trigger pack slides up inside the receiver and into that fork. Your end cap is shaped the same way and when you finally get to reassemble it, you'll have to make sure both are set facing the same way or the trigger pack will not slide back in there.
As far as rebuilding one?? Yes, it can be done, but it's a bit of a pain. That brass cap the piston shaft slides inside of has 2 o-rings inside it and one or both are shot. They're also a pain to get in and out but it can be done. Been there, done that and how I know it's a pain to do...lol. Now comes the slightly expensive part because you're gonna need a high pressure hand pump, the probe adapter, a Hatsan fill probe to refill that gas ram to the right pressure and those 2 o-rings. That stuff is gonna cost you an easy $60-$80 if you can get the hand pump on the cheap side. That or an air tank and that probe and fitting because that gas ram pumps up to 125 bar or about 1875 psi. I already had both owning a couple Hatsan PCP air rifles. From the experience, just cheaper and less hassle since you want your gas ram back to buy a new one and be done.
A new gas ram from Hatsan is about $45, should come already filled and would just drop in and you're done. If you're emailing them, use their General Inquiry form, NOT the Customer Parts form. Tell them you need the gas ram to fit a Hatsan Model 95 Vortex and how much will one cost. It usually takes a couple days before they reply, but when they do, they'll tell you if they have one, how much, how much shipping is and usually include it all as an invoice with a link to pay for it. I live in NY and any time I have, whatever I ordered is usually at my door in 4-5 days. While you're at it and have that rifle open anyway, include a new piston seal and replace it while you're at it. Here's the link to that General Inquiry formand also from experience, they seem to reply faster to those than they do anything else:
https://hatsanairgunsusa.com/contact-us/ After it's replaced, store that rifle muzzle down. That ram has a bit of silicone in it to keep the seals lubed and storing it muzzle down keeps that silicone on them so they last longer.
