[Delphinus]

I'm going to guess double-ended because your single ended lamps would likely have been running probe-start ballasts which are just the two components (transformer and capacitor) which are usually together and there's no ignitor (well there is, it's just "in" the lamp itself). Thus a ballast replacement on a single ended fixture is as simple as 'take the old ballast off' and 'replace it with the new one'.

Although it's just a guess on my part based on what little knowledge I have about how ballasts work (which I'm just compelled to share even if nobody's listening to me, because I may not know a lot of a lot of things, but I know a little about a little, and darn tootin' I'm going to pad my post count with stuff that on the surface looks helpful but when you get right down to it nobody's really listening to that Delphinus guy anyhow because he's just a nerd.)