I put a solid belt on my 1958 TR3A in 1990 when I did my full body-off restoration. That belt was from TRF and was marked "Made in Germany". It was a solid belt and only lasted 13,000 miles. These belts are hard to put on and they don't flex around the pulleys as the original ones. The original solid belts were solid and ran fine. Poor quality today. I was on Hiway 115 just south of Peterborough on the way to the Canadian Classic in Burlington at the end of July, 1993 when that solid belt broke. It took me less than 5 minutes to thread on my spare belt which was a Gates cogged belt and which measured 17.5 mm wide. In 2007, that cogged belt had worn narrower - down to 16.5 mm wide and it was slipping in the pulleys. I couldn't tighten it any more because the generator was so far adjusted over that it was hitting the inside if the inner wing. That belt lasted me 80,000 miles. It wasn't broken - it had worn too narrow. So I put on my new spare.
Now to my present spare. I bought a NAPA cogged belt with a nominal 19mm width but when I got it and measured it, it was only 16.5 mm wide. That was the same as the one which was already slipping in the pulleys. So I returned it and bought another NAPA cogged belt with a nominal width quoted as being 22mm. But when I got it, it measured 19,4mm wide at the top of the "V".
Like BOb, I don't tighten them too tightly.