Got it figured out.
We have it wrong. The cover to the TM 9-2320-361-10 has a typo. They mixed the M109 and the M50 FSN's up. so the data plate is good.
The M109A3 is a multifuel truck, and the FSN is correct.
From Wiki. See the yellow highlighted text.
M35A3
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From 1994 until 1999, the M35A3 variant was introduced as part of Extended Service Program. Usually, A3 vehicles have a
Caterpillar 3116 Diesel engine and had their manual transmissions replaced with
Allison 1545 4-speed automatic transmissions, as well as receiving numerous other improvements with a redesigned frontal appearance. No new A3 standard-transmission vehicles were produced, all vehicles being upgraded from previous configurations during the rebuild process.
The exception is some M109A3 shop vans. A small number of M109A3s were upgraded to A4 specifications using the M35A3 upgrade parts and procedures. As-built original Pre A2s are gasoline-powered, A1s use the LDS 427-2 multifuel engine, A2s use the LD/LDT 465-1c multifuel engine, and A3s use the Caterpillar diesel.
It is common, however, to find rebuilds of former gas-powered REO and Studebaker models having A1 and A2 multifuel configurations. The original contract for the M35 - Service Life Extension Program was competitive. The last two companies to send vehicles to
Aberdeen Proving Ground were
AM General, South Bend, IN and
Cummins Military Systems, Augusta, GA (AMG was awarded third quarter of 1993).