Basically, it's a mod they did to make it more voiced to the Triaxis, it added low-end. This came stock on 50's since ca. '92. So that could very well explain why your fifty seems to be voiced a bit more 'darkly'.
Seeing as yours is an '89 one, it is very possible it was simply modded, which was done quite a lot.
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Face your fifty from the front, now look along the right side of the amp, are there screws sticking out of the grating? Can you see a silver boxshaped thing behind the grating, being held there by the screws? Or do you see this when the amp is opened up? That would be the choke filter and would indicate it is deep-modded.