peerless wroteThat is Chicago Hardware Co. (not to be confused with Chicago Lock Co)Keyblank #1024
The patent for the lock has to do with a novel masterkeying concept, your lock is not masterkeyed though. The patent date(s) were stamped on all their cylinders.–Vince
Unfortunately the Old Ilco 1024 blank is obsolete, see thumbnail attached. Known substitutions are CG-9 Curtis, 133 Keil and 2604 Graham, also all long obsolete. An olde time key shop might still have a few of those blanks available.

Pete Schifferli
[unknown]