Groff is a troff implementation and as such it can be useful to run troff from V7 Unix and compare its output with groff's. This is no small task. To achieve this you can use the SIMH emulator to run V7 Unix and use troff. However V7 troff output is destined to an arcane piece of hardware, the GSI C/A/T phototypesetter (wiki/CAT) which can be hard to come by these days.
In this position, it is useful to have a piece of software which can translate V7 troff output to PostScript or any modern format. One such piece of software is cat2dit (Roff.js/cat2dit).
You can find additional information on how to run this program and other details on the groff mailing list (2021-11/msg00085).