It was/is a bayonet style lamp base. Very similar to the type of base used in automotive turn signal lamps up through the last century but with only a center contact on the base. The lamp has two small pins protruding on the sides which lock into a"bayonet" style socket.
The lamps can still be found on e-Bay. Other, smaller sizes such as M-3's could also be used with a step-down adapter. The tricky part about using lamps on this vintage camera is that there were "M" sync bulbs and "F" (focal plane shutter bulbs) sold. I think this Canon used the F bulbs only, although it also has an "X" sync for electronic flash.
M3 bulbs could be used for either M or F sync, but the adapters are not very common (they used a smaller base).
I just found out the other day that some sizes of flash bulbs (I think M size) are still made and sold in Japan for use in a stencil type printer.
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