Looks like the chain is smaller than the sprockets, i.e. a 520 chain on 530 sprockets. The chain came from "another bike" per original post.
Could be. The "pitch" (distance between pins) is determined by the "5", all 5 series chains have the same pitch. A 4 or 6 series chain would be so far off it wouldn't work a bit.
The 20 or 30 is the width. I don't know if a 20 would even fit on a 30 sprocket. Maybe a 525 could be a tight fit on a 530 sprocket. Don't know.
Chains are 'coded' in 1/8" increments.
520 chain is 5/8" x (2/8") or 1/4"
525 = 5/8 x (2.5 x 1/8") or, 5/16"
530 = 5/8" x 3/8"
420 = 4/8" x 2/8" (1/2" x 1/4")
etc.
Only 'odd' size is 428 which is slightly stronger/wider than a 425 and compares to the primary chain fitted to many (most?) vintage~1950's British bikes
