Terry, the Keeton mat cutters don’t take advantage from the notches you describe, unless you have a Hendrixon head on your Keeton. The notches serve only as a depth of cut indicator on micrometer gauge of the Hendrixon head. Normally, any ordinary blade intended for the C&H type manual mat cutters can be used on the Keeton cutters. The physical dimensions are identical.
How do I know? I was an authorized dealer and service provider for all of the above in the late 60’s and 70’s basically as soon as they came onto the market.