[stickshift]

I was going through my archives and found a note that corroborates what PJ and Bob said. This does not answer why a 1930 frame would have only one "A" though. These older trucks would have the worm drive rear axle.

"Note – For about 2-1/2 months, starting mid February 1928 (with engine ID A-26268), the AA’s used the same multiple-disc clutch spring as was used for the A chassis. Consequently, during this period, the engine ID had an “A” prefix. Refer to the “Indianapolis Service Letters” dated February 23rd and May 15th 1928. An AA with engine and frame ID of AA-98977 has been found. This is a late April 1928 number and helps define the end of the “A” prefixed ID’s for the AA’s."