1. The top plug looks like it has two separate conductors at the ring compared to the pomona, what is this? Is the Pomona plug correct or is this a difference in specs?
That extra metal "ring" was provided for long-term, heavy-duty protection in telephone switchboard use. Where those plugs were inserted and pulled out perhaps hundreds of times every day. (Back before the days of automated phone systems where the user would dial the number for themselves and relays would make the proper connection.)
The extra metal ring was not electrically connected to anything.
2. I was going to make a dual purpose patch cable (audio frequencies) for balanced and unbalanced use. Is there any benefit in using all 3 "conductors" and chassis ground the shield to the instrument end only?
Should I be using twisted signal cables for the balanced or will a double shielded coax be the thing?
We commonly use ordinary "mic cable" which is flexible, shielded, twisted pair. It is customary to connect the shield at both ends of a patch cable, and implement whatever ground-lift, etc. in the wiring between the patch bay and each piece of equipment. The whole area of shielding and grounding is a very sticky wicket with galaxial political ramifications.
That is a "long-frame" switchboard/patchbay version of a 1/4 inch phone plug.
Bantam phone plugs are a smaller variety, not interchangeable with the model you show in the photos.