Original sheet music question

I've started buying original sheet music from the '20s, '30s, & '40s for songs I'm interested in learning. I've found that they contain the very interesting intros that are usually omitted in modern reproductions and fake books. They make for a more interesting performance.

A lot of these old sheets also have uke chord diagrams. Again, some of the old voicings and patterns are more interesting and pleasing than the more modern treatments.

At what point did the "normal" uke tuning switch from D tuning to C tuning? Why was D tuning the norn? When I get a new sheet, how do I tell which tuning the uke diagrams are in?

Thanks in advance for any insights!

I cannot answer all your questions. But as a occasional collector of old sheet music myself from the same time period I see that many of the songs that I have collected have the ukulele tunings noted at the top of the page, many are in GCEA . Mr. Chadwick up in Ontario, Canada would certainly know more about this subject. Hopefully he will chime in on your thread.
Reno Dave

As with Reno Dave, I can't answer all your questions, but can supply a few insights which might help

I've seen reproductions of ukulele "method" books from the earlier part of the 20th century in both C and D tunings, and I've got at least one tune/song book from the mid '60's with chord shapes for various instruments in the back, including soprano ukulele in D, so it would seem that any "standardisation" to C has happened within the last 50 years.

As a lot of the early "methods" were published, at least in name, by performers of the era, the choice of tuning may well have been influenced by the keys that that particular performer sang in. Doubtless the sheet music of the day would have followed what was likely to have been the most popular tuning at the time, simply to encourage sales.

Of course, for whatever reasons, there are/were regional variations as well. I've read that in Canada the D tuning is still popular in schools, or was until fairly recently. That may simply be down to a supply of method books in store that need to be used before any changes are made