Oscar Adler. I checked. There are two of the 11+ Adler models that do have alternate C#/B/Bb over the F/E/D keys:
Sonora and Triumph. The Triumph was the most expensive model and the Sonora was a small step down from that. In this case, we'd be comparing the Penzel-Mueller (I'm just going to call it "PM" in the rest of this thread) against the Sonora, because the Triumph has the zillion left side altissimo keys and neither the Sonora nor PM do. The Sonora looks like it has a D# or F# altissimo key for the right hand and a G# trill key. I mention this primarily because neither the E&S System horns (like,
according to an 1899 E&S ad, they're not listed) nor the PM have either of these keys.
I did some checking in my archives: looks like the newest E&S System horns I have pics of is from 1925 (sn 28xxx). None of them have left-side bell keys, like the PM (looks like E&S introduced right-side bell keys in the 1930s). However, the
G#/C#/B/Bb cluster on the PM is identical to the
E&S. I checked all the Adler models I have pics of and
those keys generally look like an oversized version of what you'd find
on a Conn.
IIRC, there was a comment on that Beaufort thread you mention, TrueTone, that said something about the bell-to-bow and bow-to-body connectors look the same on E&S horns. They're also identical to the ones on the Sonora, so that's no help.
My current opinion, based on what I've seen, what I've posted above, and the fact that I know that E&S horns were imported to the US, is that:
A. Holton Beaufort "Imported" horns in the SOTW thread are E&S System horns. (Or, to show a specific example,
this one.)
B. The PM horn is an E&S System horn.
My opinion on the Holton is stronger than on the PM, though. I'll also note that Helen mentions that the Sonora was available with split bell keys. Then I'd have to choose which is more important in the identification: the lack of some keywork or the placement the bell keys.
It's been awhile doing that research, True Tone! Thanks for getting me to do it. It was pretty fun. I actually chopped this post down significantly, too.