Gee, that's interesting. I have a Titan II, and my brother has a MR-12. The 2nd stage are identical, but the 1st stages are different. The MR-12 1st stage was internally and functionally the same, but it was a simplified and more compact redesign of the earlier first stege. You'll have to post a photo of your first stage. I guess that the Titan II became the MR-12, but for perhaps one year in between it was called the Titan II MR-12. The Titan II came out in 1967. Your Titan II MR-12 would be from perhaps (I'm guessing) 1969. Maybe 1968.
They are nice regulators. The 1st stages are balanced diaphragms with a big external intermediate pressure adjustment. They share the HP seat with a US Divers Conshelf so it should be no problem to have serviced.