Duke I think the top one with the smaller spline is what was fitted to the 250 models (D,S,SF etc) and is exactly what I originally fitted to my experimental DC which is a 350. (first 3 pix)
I soon realized that it should have had the bigger cush drive clutch, (4th pic) and like you went on the hunt for the spider with the small spline. I was also thinking of getting one made but fortunately an old RE bloke had one which he donated.
I wonder if you would actually notice a difference without the cush?
For instance, the special I built, which is a effectively a BSA M20 with a much more powerful Indian 741 engine fitted has no cush ( nore dose an Indian 741 in it's natural form,) and I can't say I notice any difference. Likewise when my M20 first came to me 30 odd years ago, a previous owner had locked the cush drive and I rode it without knowing for a while.
Not the end of the earth to give it a try and swap it at a later date. Ron