I recently got a YashicaMat 124 G. Nice box! My brother had a Ricoh TLR many years ago (it was stolen) and since I'd enjoyed using it when the Mat came up I bought it. Quite a change from Pentax DSLRs and SLRs - LX, MX, ME Super, etc. and from the Pentax 645n kit. So, now to buy a carrier for my Beseler 23c for BW and gird my loins and do some E 6. Home processing, of course. I've done it before, I'll do it again. And those slides from the Mat ought to be magnificent through the Kindermann projector I picked up for using with the 645n.
The Yashicamat is a fine camera. I used one professionally in the 1970s. In those days foreign magazines and newspapers preferred a larger-than-35mm slide for color photos. I sold a ton of photos to Japan, Germany, Italy and other countries. I still have two 'Mats and they are still great cameras.
Yay Yashica! Mine is the earlier Yashica-D with auto frame spacing, good shutter, but without meter. I have heard that later production crank winding models need a little tenderness on the gears. Just FYI if you haven't heard it.
I have an earlier Yashica-D too with the three-element Yashikor lenses. Not as good as the Yashinons (which your Mat probably has) as I understand, but still plenty good.