After your post, Myfreeford, I went out to the shop and rechecked my work.
I disconnected the vacuum line from the carb to the distributor and read the inital timing-it was 15 degrees BTDC.

Leaving the vaccum line disconnected and plugged, I set the display on the timing light back to 0 at idle and then ran the engine speed to 2800 rpm. Mechanical advance increased to 12 degrees @ 2500 rpm and did not increase between 2500 rpm and 2800 rpm.

With the engine at idle and using a vacuum pump to apply vacuum to the diaphragm at the distributor, I increased the vacuum to 18" and got 12 degrees which is what I had set the diaphragm to previously.

Maybe, somewhere along the line someone put a really stiff spring on the mechanical advance which is limiting the mechanical advance to the 12 degrees even though the tab is clearly marked 13L. It would be interesting to know.