I think there might have only been about one year of Unics (later UNIX) development on the PDP-7. The PDP-7 Unics "version 0" is from 1969 and UNIX was on PDP-11/20 in 1970. In 1973, Version 4 Unix was rewritten in C, a language that maps to the PDP-11 which suggests that the PDP-11 was a stronger influence on UNIX than the PDP-7.
You wrote "the PDP-7/VAX model" but the lineages go "PDP-1, PDP-7, PDP-9, PDP-15" (all 18-bit) vs. "PDP-11, VAX-11, VAX (without the -11 as backward-compatibility with the PDP-11 was deprecated in later models)".
In support of some of your comments.... there was a time when older model IBM mainframes were returned and they re-appeared as disk controllers. The IBM mainframe is generally based on separate functional units closely coupled.
I think there might have only been about one year of Unics (later UNIX) development on the PDP-7. The PDP-7 Unics "version 0" is from 1969 and UNIX was on PDP-11/20 in 1970. In 1973, Version 4 Unix was rewritten in C, a language that maps to the PDP-11 which suggests that the PDP-11 was a stronger influence on UNIX than the PDP-7.
https://queue.acm.org/detail.cfm?id=3212479
You wrote "the PDP-7/VAX model" but the lineages go "PDP-1, PDP-7, PDP-9, PDP-15" (all 18-bit) vs. "PDP-11, VAX-11, VAX (without the -11 as backward-compatibility with the PDP-11 was deprecated in later models)".
In support of some of your comments.... there was a time when older model IBM mainframes were returned and they re-appeared as disk controllers. The IBM mainframe is generally based on separate functional units closely coupled.
A phone is not a mainframe, it is a dumb terminal mostly useless if not connected to a network.