There was two types of Mercury cylinder, the early ones (mk1 & 2) had a one piece cocking lever and the cylinder had the front mounting screw bracket bolted to the cylinder with one bolt in the middle, they also had pins to mount the barrel onto, the later mk3, 4 and Challengers had the bracket bolted to the cylinder which holds the articulated 2 piece cocking linkage to the rifle, the barrel is also held on with a bolt and a slotted nut, as long as it's the later type cylinder then it should fit with out any problems, the only other problem you are going to have is the trigger block will be higher than the cylinder, so you will need a standard Mercury trigger block, the Challenger piston has a slot on top to miss the maxigrip rail but will work in a Mercury cylinder (but a Mercury piston won't fit in a Challenger cylinder as it will collide with the maxigrip rail)
If you do get the standard Mercury cylinder then let me know as I have a spare Mercury trigger block I'm happy to part with.
Cheers
Pete
Ps I've just checked said cylinder on the bay of evil and it looks like an early one, which won't be any good for you.