The Mauser 66 has been introduced in 1966. Due to the telescopic system the overall weapon length is approx. 8-9 cm shorter than a standard action rifle.
Until production ended somewhere late 80´s approx. 70.000 rifles have been produced.
Some important technical changes:
In 1970 the catch lever for the telescope has been introduced and the recoil lug on the barrels have been removed. Also in this year the high-performance system (HL) has been introduced, capable to host 95mm long cartridges.
The barrel is bedded on the system and an aluminum block in the stock. Just need to remove two screws and you have a take-down.
In 1972 the safety has been improved, the so called S-safety. From this time on it was called Mauser 66S.
Here in Germany we have a couple of gunsmith still doing custom works for the M66, e.g. offering a modern single stage trigger, adding a muzzle thread for silencers or a new scope mount (swing mount, Picatinny...).
Also changing the caliber ist quite easy be changing the barrel, I own for instance M66 in .33 Nosler, .416 Ruger and new in .450 Rigby.
I used different M66 in Africa without any concerns regarding robustness of the system or reliability.