I believe it was meant to hold film canisters, used by sports photographers on the sidelines at a game. Yes, it is that old.
The Sideline Shooter is a small lumbar pack / belt pack / fanny pack (go ahead, Brits, giggle). It is lightly padded and came with two dividers, which I lost. Originally, I used it for urban outings, carrying my old Minolta 24mm, 50mm, and 100mm lenses with one mounted on my XD-11 or SRT-102 bodies. That's about all it can hold, but it was enough. I think it would be a great size for current compact mirrorless bodies with small lenses, such as a Fuji XF setup.
With the dividers removed, it is just big enough to tightly squeeze in a Canon 100-400, hood reversed, mounted to a non-pro body. It's a very tight squeeze.
The interior is roughly 6" high, 4" deep, and 12" wide. I err'd on the side of caution when measuring because the shape isn't perfectly rectangular. It is flexible enough to stretch a little.
The materials are strong... it still works like new after at least 20 years.
It has a pair of small stretch pockets on both sides, inside and out. It has a small pocket on the outside, just big enough for some batteries, memory cards, lens wipes, etc.
It has a pair of cinch straps to hold it up more tightly against you. You can loosen them and use the grab handle to swing it around to your front to get your gear out.
It sits comfortably on my lower back.
The main opening is a lid, which opens away from you when it sits on your front. It also has a zipper in the middle of the lid, in case you want to grab something smaller from the middle section with more privacy. I never used the zipper.
I'm not convinced the padding is strong enough to protect a lens from a drop. Maybe, maybe not. I never dropped it.
