[mikethedruid]

Looking at the model 20 schematics, it looks like 45s could be used in the push pull output easily enough by changing the filaments from being in parallel to being in series. The 2.5 V filaments would then add up to the 5V of the winding for them in the power transformer. Whether this was ever done or not, I don't know. It certainly COULD have been a WW2 mod if a repair man had 45 tubes in stock but did not have 71As. More likely someone just stuffed in some tubes to make it look "full." Poses an interesting question though.