I had a SeaPro AtPac, looked very similar to the ones posted here except mine had a full jacket(not jacket BC) with pockets and d-rings. And no plastic shell.
I bought it in 81' and dove it till about 94'. Once I had the inflator break while getting ready to submerge. When it happened it was a comedy of errors, the pack started inflating faster than the purge could let out the air so I started swimming back to the boat with one hand on the weight pull thinking if the bladder pops, I'm going to blow 20 lbs. of lead shot and marbles all over the reef in Maui and not wanting to do so. Luckily I made it back to the boat and shut off the air.
And as mentioned previously, the lead shot had a tendency to settle. I mixed mine with marbles and shook it each time before I dove to loosen it up. Mine also had a nifty yellow pull handle mounted in the front of the jacket for pulling the pin on the shot pocket.
The best feature of mine with the jacket was that there was a 6" wide velcro band across your midsection. The way the jacket was designed that was all you needed to secure the pack, so by slipping my arms in the openings and closing the velcro, I could be ready to dive in about 5 seconds. I still miss that when I'm fumbling with the straps, clips, cummerbund etc. on my Ranger.
It also had one of the worst cam tank bands I've ever used, they were velcro with a smallish flat metal buckle. About 1/2 the time I'd bend a nail trying to get the d!!! thing to release post-dive. The only redeeming quality about them was that they were completely removable, there were fixed metal tubes mounted through the plastic shot pocket and the tank brackets had bolts that went through the holes with a big plastic wingnut to secure them. So when you removed them, you could fold and store the pack in a carrier about 3" wide by the l/w of the pack.
Mounting doubles on this pack was easy, the mfr. sold a doubles bracket with the same fittings. And since it was a hard plastic backpack, they were mounted solid.
I sold it to a former employee of the company about 5 yrs. ago who still uses one. A few years ago I found a company on the web - I think near Seattle - who were still selling the component pieces, I think they bought the rights when SeaPro folded.
It was a good pack for it's time. It was an early adaptation of the BP/W design. When everybody else was fussing with weightbelts and BC's with straps, it was a pleasure to don and use.
It had the problem of floating you face down at the surface if that was the position you surfaced in since all the weight was in back. You could compensate for this by carrying lead blocks in the front pockets, they had individual velcro releases.
Attached is a picture from sometime in the late 80's/early 90's. You can see the horsecollar bladder in the back and the yellow quick release weight handle. The jacket is a bit obscured as I'm wearing a black t-shirt. The reg is a Dacor Pacer 900XL.