If your transmission is a genuine Allison Reman, the serial number will start with "R" or "RM". These units have a different lookup path because they have a standardized parts configuration across multiple model years. Never assume a Reman unit has the same internal upgrades as a factory unit of the same model number.
On larger units, look at the lower rear face of the rear housing. Allison Transmission Serial Number Lookup
If the rivet tag is torn off or rusted, go to the machined flat surface on the lower right side of the main case, just above the oil pan rail. If your transmission is a genuine Allison Reman,