Brian Gaff presented the following explanation :
Yes even in the 1970s you could by fairy lights all wired in series across
the mains, usually 20 bulbs with one fusible bulb which had bayonet connector
on one end.

You bought the replacement lamps by describing the base and the total
number of lamps in the set. I guess they were all the same wattage,
just the intended voltage which varied.