The "12M" (low Bb) and "11M" (low A) tubes were still running in the late 90's. UMI made Conn 13M (low A) and 14M (low Bb). The were more or less based on the old tubes from the mid 30's/late 60's. But the keys(works) were new/modern. I guess the made some improvements on the tube as well to be able to play a modern mpc on thier baris. Some of these baris had rolled tone holes. I guess it was more a "Hey, look at us and see what we can do" gimmick. The entire UMI saxophone production (Conn, King and Armstrong (not Musica)) was in Nogales, Arizona, USA (former Art Best Manufacturing Company that Conn bought ca 1960). The town Nogales is devided in two parts, Nogales, Arizona, USA and Nogales, Sonora, Mexico. Most of workers in saxophone manufactoring came from Mexico. And they were skilled. Maybe the parts were made in Mexico and put together in USA (like Selmer MK VI, made in Paris and put together in Elkhart). I don't know?
The letter prefix on Nogales/Eastlake saxes is not clear. I have seen saxes that were sold/made in the late 80's with the prefix "N" which indicate that the horn was made in 1969/1970. I think a "N" sax can be "younger". I have never seen "P" and "R" prefix on a Conn sax (I think the "P" older "Conn" second line and stencils indicated that they were made in the Pan-American plant in Elkhart owned by "Conns" parent company).
Thank you for an interesting review. I don't think "Mexico" saxes are impossible. The prices should be low and seller should stay away of words like "vintage" or "12M" even if the sax is based the tube. But you can do bargains in these segment. I helped a friend to buy a mint "Bundy" baritone based on a "Buescher 400" for nearly no money at all compared to other low priced baris on the market. And it has a big mature bari voice as well. Not the modern "basson-like" baris. But this is my personal opinon!