This question actually came in now several years ago, and I answered it by e-mail, but it is worth noting here in Horn Matters as well.
The question had to do with fingering G on the second line on a five valve single Bb horn, such as the one in this illustration, with mention in the question also of a few famous players of the past who used single Bb horns.
Besides the three regular valves on this horn there are two more, one is a stopping valve that adds the same tubing as would 2nd valve on the F horn (more on stopping valves here), and the other most commonly referred to as an F extension. It would be the same length as a 1-3 combination, but longer enough to actually be in tune (as 1-3 will be quite sharp).
On a double horn your only viable option for that G on the Bb horn is first valve, but it will be somewhat flat. On a five valve single horn the F extension would be ideal to use, if it does not slow down your technique in any given passage. The general idea is you can tune that valve to be perfectly in tune for the G, avoiding the somewhat flat first valve fingering.
But in faster passage work you would likely still want to use the first valve. It may be a bit flat, but if it goes by quickly nobody will notice.
In my own case, on my single B-flat I don’t have the 5th valve F extension, and normally use the stop valve with extender slides so that it is just a bit shorter than the first valve. I have seen photos of Peter Damm that say to me he also used this tactic. It only works if you don’t have stopped notes to play. I use that tactic on my descant as well. See the photo in this article:
Finally, a disclaimer, some horns are better than others in terms of that note being flat on the first valve, but it is a problem on most single B-flat horns and descants. Good luck!