I have a couple of manifold rubbers that are not RM125S but could fit given those dimensions. One is off some sort of 77/78 RM, I cut it down to use on an RM125M with a 34mm carby. I think though it doesn't fit because it is stepped, ie the carb bell would need to be below the airbox outlet, but look, you never know. The other looks like a better fit - it's straight, but quite short, only 35mm long. Carb end 58mm and airbox end 72mm. Could be Husky I suspect.
You're welcome to both if you like.