[Arran]

It may be a different model, normally a manufacturer would come out with a different model number even if the pushbutton bank was the only change. Anyone could have changed the eye tube, many will swap a 6U5 with a 6E5 though they are not directly interchangeable. A 6N5 is interchangeable for a 6AB5, you can use a 6U5 in it's place but not the other way around. The 6N5 was intended for use in farm sets apparently so it runs on a lower B+ voltage, it may or may not be correct for this set.
Regards
Arran