Welcome to the forum.
You did not say in your post that your Winchester model 63 with serial number in the 159000 range had the grooved receiver.
I'm guessing that it does.
You do have a late M63 and if the machining and finish of the groove looks correct, I think you can assume that Winchester did it.
A manufacturer may not start a change at a precise SN and every gun after that have that feature. You might find that change before and after a specific SN.
You might want to go to Rimfire Central: http://www.rimfirecentral.com/forums/
and go to the Winchester section and post your question there.
Those 63's are neat little guns and well liked and shoot great.
Hope this helps.