I didn’t think to look in the Staufer books – one tends to view then as a tertiary source – but they nevertheless do sometimes provide additional information.
The E2b and P5A case is an interesting one. Here is a pertinent picture from Solomon, “Electric Locomotives”:
Solomon p.68.jpg
It shows the E2B as having four MU sockets on each side, the outermost being marked “P5A” only. This picture could have been taken at any time in the operating history of the E2b fleet, so did not necessarily reflect its original configuration.
But here is a page from the Railway Gazette 1952 April 18 article on the E2b. It is highly probable that the picture shows the locomotive pair in “as built” condition. The same array of four MU sockets per side is visible, although their labelling is not. I think that that points to P5A MU capability being an original feature.
RG 19520418 p.433.jpg
Whilst retrofitting was certainly not impossible, factoring in mixed MU at the design stage would have been easier. Thus the new locomotive design could have been configured with the same number of accelerating notches as the older design, and with the same auxiliary electrical system voltage. And as said, it would be difficult to rationalize the 32-volt choice in an era when 74 volts was the norm other than that it was for backward compatibility.
So I’d say that the preponderance of evidence supports the notion that the E2b could MU with the P5A as built.
Re the E44, it was the lack of mention of diesel MU capability in the Operating Manual that made me rather sceptical. Given that the OM was railroad-specific, it is not as if it was restricted to covering the basic model only. But then it was numbered GEJ-3809B, and covered both the ignitron and silicon rectifier builds. That suggests that there was at least one earlier issue. Just possibly diesel MU capability was an original feature on some of the earlier build, but discontinued and possibly removed before the last of the fleet were built, and so not included in the late edition of the OM.
On the steam-to-diesel MU case, as I recall the 1970s operators of ex-SP #4449 had a portable diesel controller that was used for that purpose, and I think that they claimed to be the first to do this . And in Victoria, Australia, one of the R-class 4-6-4 locomotives in preservation was fitted with a diesel control stand. This combination appears to have been a preservation-era development. Westinghouse is reputed to have developed MU systems for steam locomotives, although evidently never deployed. The closest approach to steam MU was probably found in European push-pull passenger train operations, where a trailing steam locomotive (mid-train or at the rear) was controlled from a leading cab-car.
Cheers,