I have taken the transformer apart and it looks like a Magnequest.....In side is written in ink....5K@20ma to push-pull grids.............
Sometimes..a transformer will fail at the internal solder junctions. so magnequest it is ...and... how’s the impedance measurements at the solder junctions for the wire lead-outs? It’s the ’last chance’ test for a transformer, generally speaking...
Of course, we don't know if it is a failure you are dealing with or a desire for more transformers of the same. If you called in with a Magnequest part number at a Hammond corp help line, well, that might explain it...but I think you figured out that aspect already....