I'm familiar with the shape of the key he's describing, it's normal, yet not as common as the regular round ones.
The ones I've used and bought are for the cam and balance pulleys on a Detroit 53 series.
Measure yours and I'll see if I have an old one to measure. If the same I'll dig up a part number and point you in the right direction to buy one.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-