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But they rarely are the newest version.I think the easiest way to get one is to buy a 2 port MK5. They're pretty cheap.
Yep, Chile is looking for the stepped piston stem version. It was an upgrade that supposedly prevents the piston from going too deep into the hp seat. If the piston travels too far, it could plug up and stop air flow or extract the seat from the seat retainer. I've never heard of a case where this has actually happened, but if the spring were to break I suppose it's a possibility.
That's why I suggest using a bushing over the stem if there is any concern. Here is a thread with pictures and a discussion.
http://www.scubaboard.com/forums/repairing-your-own-gear/303673-sp-mk5-piston.html