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If I had a hundred bucks for Rule's book, I'd know the answer to this question! What year di Winchester start drilling and tapping the rear bridge? I'm guessing that's a transition feature, just looking for the first year. Thanks.

Good question...anyone !?

Well Keith... If you had Roger Rule's book, you'd spend some time sorting through information and the best answer as you note, with the introduction of the Type II, at SN 60500, aka Transition model in 1947. Yet exceptions! The long mag models generally about a year later. Then too, the factory offered D&T as special orders. That said, with the caveat nowadays that all earlier than normal production D&T receivers are presumptively 'aftermarket' non-original.
Largely a R.Rule recited...
My take

Thanks, iskra. It appears that d&t falls in line with a common theme of pre-64's, that being "mostly", "generally, with exceptions", "not always", etc.! I guess unless you were in the factory peering over someone's shoulder, you'd could never be 100% sure of what, where, and when! I'm sure there were many years of production that are chock full of changes thst really didn't fall into any calendar dates; if they had realized the icon status of what they were creating, maybe us poor slobs in the future wouldn't be scratching our heads quite so much! I'm good with 1947 on d&t.

My 1947 transition mod-70 is d&t, factory ? I would like to think so, but if not it is still d&t