I got curious about exactly when the modern T-frets were invented after working on refretting a neck with bar frets. After a bit of Googling, I found the first patent of T-shaped frets of John F. Stratton, US Pat. no. 501,743, from July 18, 1893. Patents are not always taken by the original inventors or those who first used what was patented, so it is not incredible that similar T-frets were used a number of years before.
The Bay State guitar in current batch dated to circa 1888, which has T-frets that appear to be original - in any case, the fret slots are not as wide as they would have been with bar frets, may have had some of the first T-frets that were manufactured.
The modern frets with crown and barbs were patented by C.F Smith, Pat. no. 1,727,620, as late as 1927. I also found one new cool patent from 2010 with more flexible frets that should be manufactured and sold!
I took some pictures and measured the frets on the Bay State guitar from circa 1888.
They are similar to the fret in the patent, except for the longitudinal "barb" in the patent. I think the last one was what was needed to get the patent approved, and not very practical in practice. The Bay State nickel frets have instead been scored on the underside with a knife or chisel so that sharp bristles stick out and help to hold the fret in place. The frets have roughly the same shape and the same flat crown as in the patent.
The measurements of the low and narrow bands are:
Total height: 2 mm
Width of crown: 1,4 mm
Height of crown: 0,5 mm
Tang thickness: 0,65-0,7 mm