Silca frame pumps weighed a whole lot less than a Zefal did, and that's mostly why they got used. Also they were frame fitting before Zefal offered that feature with the HP-X. They pumped up your tires a bit faster too, but more coordination and arm strength was required. Campy metal heads were pretty much required. The campy plastic heads were OK but the silca plastic heads sucked. I wish I still had my old Silca but it got stolen at some point. I do have the Zefal HP I've had since I was like 12, and I concur, functionally the Zefal pumps are better. Dead reliable and easy to pump. HP was the same as HPX but not frame fitting.

Along seat tube was more or less standard, but along top tube also was popular. The latter requires some sort of strap or else the pump flies out when you hit a bump.