Mike, that's very interesting about the tap levers...I've often wondered if the older lever would work on a newer gun. It is certainly nicer-looking and more ergonomic, wonder why they ever changed it?
Without pulling out my trusty Hiller, I think the serial number would put the gun 1970-ish, give or take. Hiller's book also contains great directions for dismantling the Mk 3; the only real trick is the backwards-threaded lug holding the rear section on. The Mk 3 is pretty idiot-proof (translation: even I can take one apart without harm to gun or mechanic).
BTW another FANTASTIC book for working on airguns is "Air Rifle and Air Pistol Maintenance and Repair," by Mr Q Cobham (seen near the top in this equally fantastic post from Grant's forum):
This is a current item and I believe you can still get it via Amazon or directly from the author (sometimes he sticks them up on Ebay, which is how I got my copy). It also has superb information on the Mk 3. In my humble opinion just about anybody who frequents this forum ought to own this one!