If you mean twenty-five pellets packed head-to-tail in a .177" bore tube, one thought that occurs is pellets pre-packed in a speedloader to be used with the Gamo repeaters (ASI Paratrooper and Sniper repeaters). The date would certainly be right for them having been introduced, in Spain, although quite when they started to be distributed in this country I don't know - certainly by the late 1970s.
(As I understand it, it is possible to remove completely the magazine tube on these guns for loading, rather than it being fixed in place and pellets inserted one at a time through a loading port.)
Edit - I don't actually know whether Gamo made such a thing, though.
Iain