That's to indicate the front of the engine. Here's a cross-section from the 1955 2nd Truck Shop Manual that shows the notch.
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235 cross-section.jpg (45.83 KB, 280 downloads)

Last edited by klhansen; 05/09/2025 12:45 AM. Reason: added picture
Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.