You could be describing an Admin Sep Board(for enlisted), or a Board of Inquiry(for commissioned officers). I didn't enter the JAG Corps until 1985, so they could have been called something different in the early '70s, but I don't believe they were.
Do you remember if the object of the your "panel" included the separation of the service member? If that's all that was at stake - no confinement, no reduction in grade, no forfeiture of pay, then you're definitely describing a Separation Board or a Board of Inquiry. They can certainly (and frequently will) be called based upon the conviction of a service member in a civilian court of law. If there's a criminal conviction in a civilian court, Admin Sep Boards are pretty straightforward. There's not much to argue, so it wouldn't appear adversarial, in the least - the service member was either convicted, or he wasn't. There's no "prosecutor" either. There's only the panel of officers, the service member, and his counsel, should he so elect (at his expense).
But, if your service member was facing additional charges that stemmed from a civilian conviction, Like an Article 86 spec, then at minimum, it was a summary court-martial.
There most definitely were three JAG officers, each with his own role to play. One, a red headed guy, was the "defense", another was the "prosecutor", representing the convening authority, and one, a slightly chubby Jewish looking fellow, quite bored most of the time, sat off to the side, he was described as the legal adviser for the panel/court, certainly he was the one who laid out the requirements of the regulations UCMJ, and what our options were, and were not. Since this was regular court room, he sat in the what I presume would be he judges spot, and occasionally chimed in, or answered questions that the members of the panel had. All the JAGs were Captains. The panel, all officers, IIRC, at the "defendant's choice. (He had that much good advice from his JAG representative.)
The service member was enlisted, probably was an E-3 or E-4, but I don't recall. Skinny kid, not the best military bearing.
I don't remember if one of our options was to recommend a BCD or not, but separation from the service definitely was an option. The kid wanted to stay in. His supervisor spoke well of him, his first sergeant, well he didn't show up. One of our options was a sort of discinpliary baracks kind of thing, where he'd spend his time when wasn't on duty, and he'd be restricted to the base. We gave him that, but let him stay in the service. I think the Colonel, the senior officer on the panel, would have taken a bite out of our asses if we had done otherwise, but there did seem to be general agreement. Although we did a secret ballot, I don't recall how that went. Don't know if that was required or just our option. The other piece was that our recommendation was binding on the convening authority, unless he chose to do something less stringent. He didn't.
But you're right, there was nothing to the finding of facts, they were stipulated, and documented, he was convicted of a civilian crime, well not convicted, he plead to a lessor offense, thinking that was a "good deal", but it wasn't because he was military.
The thing was, the "prosecutor" was almost as supportive of the "defendant" as the "defense", but from a different perspective.
You have to understand the times. The military had not yet recovered from the excesses of the Vietnam era. Moral overall was not that great. I was amazed how much it had changed, even in the reserve forces, when I went back to active reserve status, after a 4 year break, and even that was a bit before your time. By the time you came on board, during the Reagan era, everything had changed. I could see it during my IDT in Texas and my annual tour in Ohio. There was an energy that had been missing in the mid 70s. Before my break, one of the two troops that I rated, in the Air Guard, was the last "draft induced" enlisted member in the unit. He didn't want to be there, wore a wig to drills (that was allowed, which shows had bad things had gotten. All the officers, about 8 or 9, were past their obligated service, even if they had been draft induced in the first place. Same for the NCOs. Things were almost the same at the active unit I served in as an IMA reservist, right after I left active duty.