Yashica ML 28 mm really tight on Contax 139Q

Welcome meghatas ... Unfortunately Dropbox no longer offers sharing of direct links to individual files for forum users, but the folder links do work as an alternative.

As biggles3 noted, this isn't usual behaviour. I've never had a mounting issue with any C/Y lens from Yashica or Zeiss -unless- the lens had taken some type of damage or something came loose. It may be slight and might not easily jump out as the cause.

To follow along with the question on whether the aperture is currently active and working properly... that would help to rule out at least some part of the possibility that the linkage levers/post/brackets are the root cause. With the lens off the camera, does the aperture click into all f-stops, and will it close down as expected by moving the actuator lever back and forth - just like the DoF preview would do it? Hopefully it does, and does it smoothly. There's the possibility that the aperture stop post (or L-bracket, in this case) is bent or has been knocked out of position and is binding on something. If there's any heavy wear marks or anything visually odd to get a sense of that happening, that helps to narrow down the cause. Anything, anywhere, that looks out of place can offer some tell-tale evidence of what's wrong.

Another thing that comes to mind for ML lenses, is the amount of clearance each of the 3 bayonet tabs have, and if there are 'burrs' or misshapen edges or corners on the tab's leading or trailing edges. Unlike some of the Zeiss lenses, the ML's have a one-piece mount that's milled to form all three tabs and all the ridges, lock notch, etc., and if a lens were to be dropped on one of those edges or corners, it could bend a fraction of a millimeter and bind up on the inner bayonet flange of the camera that locks the lens in place. The recessed ring just under the edge of the tabs is 2mm in height. If you have a wooden popsicle stick, that's 2.1mm in thickness, and can the pushed into the 2mm gap and peel back enough wood to give you a gauge of sorts. That can be inserted at different points around the mounting ring to see if it's deformed somehow. If it's less than 2mm, it can be 'relieved' with a little gentle persuasion. :) A round toothpick measures 2.03mm in diameter at the middle, so it can offer another makeshift gauge, but maybe not as consistent or easy to judge with.

Sometimes images may not offer a critical enough view with precision machined parts, and some form of measurement is needed to diagnose precision related issues. As you might guess from the figures above, +/- 0.03mm in difference is enough to cause some difficulty in getting the tabs to slide in fully without friction or binding up.

I would be remiss if I didn't add that gunk and grime can solidify over time and turn into a 'glaze' that can be overly thick, and/or cause enough drag and friction to make it difficult to get parts matched up and locked together. It would be great if a simple cleaning works, but if not, it offers a chance to inspect the parts while doing it.