Other than a builder's plate on outside of unit, where is the GE serial number located on GE 45-tonners and similar industrial units?

Raymond

The 5 digit GE serial number is almost always stamped in the loco's cab or frame UNDER the builder's plate. If the plate is missing, get out your wire brush and scrapers and go to work. That's the good news. The bad is that this stamping is often quite light and if there's a lot of rust there, you may be SOL.

In addition to the 5 digit serial there is an additional stamping on all the industrial units that can lead to its identity, but it requires a secondary source of information. Each such loco had a "Batch Number" stamped in the end of the loco frame above the front coupler pocket. This stamping is generally quite deep and it takes many coats of paint to really hide it. Each group of such units--GE tended to build the common models in groups, or "batches"--had a specific 4 digit ID number. This was then followed by another one or two digit number that places that particular loco in its sequence among the others of the batch. From all the examples I've seen, the sequence numbers in a batch are in the order of the 5 digit serial numbers. The whole trick here is knowing which batch applied to which group of 10, 20, or 25 serials. If you find this stamping on a loco and don't know the 5 digit serial, report it here and you may find someone who does know the correlation for that particular batch. Several data hounds have made this a life's work.

Good luck.