Mark 4 were the most widely used but Mk 3 are still out there, often from the 60's although I don't know when they were first used. Brand New Mk4's are available for the discerning user with something to swap.
They were designed to be very resistant to sarrupticious entry-and were! Many of the cabinets were easily drilled but there were also very good safes that used the manifoil in which case they were a real challenge.
Anti Xray and manipulation (both feeling and listening), they did the job.