Yes, I'm fairly certain that the mirror lockup feature was provided primarily so that the 21mm f/3.3 lens could be used with its dedicated accessory viewfinder. When I purchased my last TL Electro, a black ITS model, the seller very kindly included a copy of Yashica's own 45 page glossy booklet "Yashica TL Electro-X The Creative System of Photography" in excellent condition. It goes into some detail about the 21mm lens and its use. It's an eight element lens in four groups and protrudes substantially into the mirror box when mounted (quite close to the film plane from the look of its side profile).This feature is handy when I use my 21mm f/3.3 wide lens.
For those who have never used this lens, it is outstanding...
Cheers
Brett
