I happened to run across Tony Thompson's blog "Prototype Lettering, Part 2".
In comparing the figures that are presented as "AAR" vs. the CB&Q ARA ST'D
LETTER set originally drawn Chicago 10-05-09 and retraced 9-23-22 (on
account of the original tracings being destroyed in the Chicago HQ Fire in
1922) the SP M, C, B, 8, 7 and 2 are identical and must be the ARA original.
Note that the letters Tony presents below the AAR letters on graph grid,
while similar, are of different proportion.