John,  I have carefully measured and photographed quite a large number
of failed celluloid Ampico A Tempo plates, and I have generally found
somewhat different dimensions than those shown in your drawing.  Most
importantly, I have almost always found somewhat smaller openings at
the low-Tempo end of the slot, where the plate should restrict airflow
the most, and where the dimensions are most critical to the function
of the Tempo regulator.

Getting precise design measurements from an original plate is a little
tricky, because the celluloid can shrink and warp quite a bit with age,
making the opening larger than originally designed.  Specifically, my
dimensions are that the overall plate should be 0.700" wide, 3.750"
long, and 0.060" thick.  The bleed hole is usually a #36 drill (0.106")
centered 2.250" from the right edge of the plate.  The low-Tempo hole
is a #20 drill (0.161), centered 0.300" from the right edge of the
plate.  The widest left edge of the slot is 0.375" wide, positioned
1.800" from the right edge of the plate.  The narrow start of the slot
is only approximately 0.050" wide, flaring with more curvature than
shown in your drawing.

The replacement plates I have made to these general dimensions have
functioned well, allowing for good regulation of actual Tempo, with
near-perfect correlation of Tempo settings with actual air motor
speeds.

Best regards,
Ralph Nielsen
http://www.historicpianos.com/