I have found a car that seems to have the same unmarked trucks that you took a photo of. These are roller bearing trucks with longer coupler boxes:
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I also found one where bolster is blank but the bottom of the coupler box has MICRO TRAINS molded into it:
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Both of these however still had .540" axles in them. I suppose it is possible that someone reamed out the bearings in your trucks and fitted the longer axles.