Man, I'll say!
My back pack was plastic, I remember that it always dug into the small of my spine on the surface, so it must have been from the pre-contour days. Doubles were a challenge since the back pack was molded for a single tank, but with a few adaptations they could be forced to work. All long before the development of the isolation manifold, so we only had two valves to fiddle with. I still remember the excitement my first doubles dive and having what seemed like an unlimited supply of air with 144cf in double 72s, now we're doing dives with over 400cf and wondering where it all goes.
The horse collars were a blast, all too often literally, should you accidentally pull the pin on the CO2 cartridges. What are friends for?

Otherwise, they worked better than nothing but that oral inflation was a PITA. I don't remember being heavy enough to need inflation at the surface and mine had a crotch strap that kept it nice and low on the neck, anyway.
We used to carry some ditchable weight with us on shore dives so that we wouldn't have to compensate so much for the wetsuits crushing at depth - a couple of bricks of lead that we'd leave on the bottom at 30 or 40 feet. One result of this foolishness was that our navigation skills were highly developed, lose that 4# of lead and you'd be scrounging for rocks for ballast. Heh.
Buddy breathing. Now there's a lost skill. Thankfully. I remember thinking that octo's, like SPG's, were a silly waste. I bought my first one when I realized that it meant I wouldn't have to be swapping spit with my brother and father, who were my primary buddies back in those early days.
I loved my 1/2" beavertail farmer john, which of course had no stitched seams, so just about every dive meant a little time spent with the glue pot, resealing the busted seams. I have no idea how many times I glued my fingers to my wetsuit, trying to manually clamp a seam together long enough for the glue to set.
And then there was my Voit mask. I've had cars with smaller windshields in them. The thing never leaked until the rubber started to fail but I think it could have held the better part of a quart of water.
Speaking of feeling old, as a teenager I remember that your knife was supposed to be longer than your ahem but not nearly as hard, when did things get all turned around? I still have my original Voit knife with the bright orange handle with the peen and bottle opener and the monster chromed blade with the serrations, line hook and sharp point. Mike Nelson had some serious competition I first strapped that thing on my leg, let me assure you.
Of all the gear I bought back in the early 70's there is only one thing that I still use - a trusty pair of JetFins that are still going strong. Amazing things, actually.
Ah, memories. Polishing them today is almost as fun as living them was.