Martyn, I asked myself this question many times and I don't know if anybody has conducted a systematic research in this area. From my own experience, I came across clearly original T-shape frets exclusively on French guitars from the early 1820s of which at least two makers are worth of mentioning: Lacote and Mauchant (et freres). The first was one of the most innovative French guitar makers, the second is from the ancient "cooking pot" of French stringed instrument manufacture. I would still probably give priority to Lacote (or Etienne La Prevotte for that matter) but it's hard to be 100% certain because most of the early 19th century guitars were re-fretted later in the century or indeed in the 20th century with somewhat larger size T-frets.
What is also interesting is that there seems to exist some sort of intermediate (to bar- and / or T-shape) frets which are of wedge-shape in cross-section. I saw these on some of what I would call "Stauffer" style guitars (made by Stauffer himself, his firm or the makers under his influence) which again can be dated from 1820s - 1830s. So the whole matter doesn't seem to be, as you say, straightforward ... As for your: > Clearly it would be most unlikely for a guitar which had been refretted with T section to be later fretted with plain strips (I mean in the 19th/early 20thC), ... Do you have any particular instrument in mind? In general, it is not at all impossible. I was recently refretting a late 19th century Martin guitar with bar-frets because it had unoriginal T-shape frets! ... Alexander To get on or off this list see list information at http://www.cs.dartmouth.edu/~wbc/lute-admin/index.html
