Does anybody know if spare parts from the Mark III will fit the Mark IV?

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Witzell
"The dude abides".

All the trigger parts are the same except the sear.

Depends. What spares? Mags, Hammer bushings, sights, yes.
Internals , Some of the trigger train is (at least one way) interchangeable. However they often need to be changed in groups vs individual pieces or modified slightly for guaranteed functional. ( Spent many an hour with an early MK IV trying to get MK II parts to work.) Triggers and sears differ slightly as do the trigger spring as I recall. It's hard to get more specific without knowing your goal.

FWIW, I found it a trickier process ( involving more parts) to do a trigger job on my pair of MK IVs as on my MK I/II/III collection. I ended up putting in the full Volquartzen kit in both. The earlier models never got more than a sear change and some polishing to get under 3#.

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Dave

The grips are definitely not interchangeable.
No previous Marks will fit the MK IV without modification.

Yea, I'm still surprised that a year after introduction, there aren't more aftermarket grip options out there for the MK IV's. Not that the factory grips aren't both excellent and functional.

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Dave