I don't think it works for the 250, it just so happens that the CR and TM125s share the same bore/stroke so the pistons are interchangeable with a little careful trimming. I am no expert on this, but I think the 74-77 CR pistons fit the 73-75 TM/RM125.
My collection of pistons include TM, TS and CR, all of which differ slightly in length of skirt, height of crown, and cutaway. I am sure there are slight differences in power as a result, but given that these things don't make much power anyway I don't think it matters much...