Im sure someone will tell you but for now---according to july 2007 airgun world, the sixth series of the mk3 ran from 1966 t0 1975 and the numbers were A0060--A9999
B0001--B9999
F0001--F6200+ so yours is closer to the first run.
Dont know about guide but strip is very easy ---take out of stock,take off cocking arm and under lever--Take breech plate off and remove tap. Take out trigger screws and then pins to remove trigger and sear--UNDO LEFT HAND tHREAD SCREW-Unscrew trigger block-- Remove mainspring and piston----(hard bit coming up) I think earlier ones had the piston washer rivetted on and later ones screwed---Sometimes,as on all guns with a countersunk screw holding on the piston washer, The screw gets battered and can be hard to remove, usually resulting in a damaged piston washer. If you are lucky then the washer will be in good nick. I think its pot luck---Ive seen some guns around 45 or 50 years old that the washer is okay on and others where it has rotted away, probably due to too much oil being put into the cylinder. I hope this helps.