If a new drum doesn't have a groove just inboard of the reinforcing rib at the open side for the spring to fit into, it's possible to create one with a die grinder and a flame-shaped carbide burr by mounting the drum on a front spindle and slowly spinning the drum by hand against the burr. Wear safety glasses and gloves- - - -those shards of cast iron coming off the burr are sharp! It doesn't take much of a groove- - - -maybe 1/4" wide and 1/8" deep.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!