Well, someone else answered it for me here.........but yes, the <EP> is for "Exchange post" which were items sold to US Military in Asia (mostly for Tax reasons in Japan) from about 1951 to 1971
Not all lenses that have this symbol are valuable or uncommon but the f1.5/50mm "Japanese Sonnar" the lens that this thread was started about with this symbol on it is truly Rare, with very few made, that`s why I received a pretty good amount of money for it on a Very Rare Canon IIF2/Rapidwinder combo and it did go to a Photographic Museum........
You know it`s funny, but Canon production of the f1.5/50mm doesn`t even come close to the Nikkor f1.4/50mm numbers
(I forget what they are but the Nikkor production dwarfs the Canon number, because of all the Tower/Nicca contracts)
...but the Nikkor is now like almost 3 times the cost of the Canon, and the Canon`s the rarer of the two (and the Better) in my opinion - my (2) Nikkor f1.4`s (a 1950 and a 1954 one) don`t get half the use my Canon does, I just like the Canon`s look better, less contrasty than the Nikkor and it works better in both color and blk n wht mediums in my style of Photography 😉
Tom