HI,
I finally remembered where it was and have looked in my July 1935 copy of "Leica Accessories" a little advertising brochure. It lists the ETRIN, ETTWO, ETTRE, ETNEU, ETGUS and ETMAX. They are all shown in photographs except for the ETMAX and so I wondered.
Did a search and got the usual confusion but I'll recommend it just the same as your motivation will be higher than mine. And I'll wish you luck.
Regards, David