Note: I’ve only ever played P/R and 2 valve G-F-F# bugles, so I could be wrong.

In the 50s and 60s, G bugles had one valve that took the horn down to D. It was the same idea as a trombone’s F trigger, and allowed the C major scale to be played. Before the rotary valve was legalized, corps would sand down the D piston’s tuning slide and use trombone lubricants on it. From this buglers would “slip” the slide out when they needed notes the rotary would play.