Lots of people mistakenly call the 109 the R109. It's actually just the 109. It and the 108 and 156 had no letter prefix. My first regulator was a Mk2/108, followed by a MkV/109 the same year I finally broke down and got certified, in 1972. Local dive shops were starting to require a C card to fill cylinders. I still have that regulator, along with several other MkV/109 combos. I converted my last 109 to a 156 and its MkV to DIN, and still use it. My first letter prefix reg was a Mk10/D350 that I used on my last Caribbean trip.I've always called my first regulator, bought in 1972, still in operation today, the R109. I don't know about the 108 or the Balanced Adjustable
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