Way back in the day, small double "50"s with a single outlet manifiold were common. I had a set that was 21 inches tall with a 2012 psi service pressure and a set that was 18" tall with an 1800 psi service pressure. Both held around 45 cu ft at the service pressure and around 50 cu ft at a 10% over fill. I eventually broke them up for singles my son when he started diving and they ended up getting sold to other divers with kids starting to dive.
It's hard to tell for sure though exactly what you have. To find out you could:
1. Fill one with water and measure that volume of water either volumetrically or by weight if you have a really accurate scale, or Fill the tank all the way, screw the valve in with the valve open to displace the extra water and measure the volume of the water in the tank, or compare the pre and post weights to see how much weight in water was added and convert that weight of water to cc's to find the volume. This would give you the most accurate measurement of the tank's capacity, but the tank gets wet and will need to be dried properly to avoid flash rust. Alternatively,
2. Fill the tank, transfill into another nearly empty tank of known volume and note the pressure when they equalize. If you know the starting pressures of both tanks with a high degree of accuracy and you know the ending pressure, you can do the math to determine the volume of air that was transferred. This only requires a digital gauge that is ideally accurate, but at a minimum consistent within the range of pressures measured.
In other words if you filled into an reasonably empty (500 psi or so) AL 80 (77 cu ft at 3000 psi or .02566 cu ft per psi) and it gained 635 psi, then you know you transferred 16.29 cu ft of gas by taking 635 times .02566.
For example, if the unknown cylinder lost 745 psi, then you know that 745 psi equals the same 16.29 cu ft, or .0219 cu ft per psi. Take that number times the service pressure and you'll have the capacity. In this example, .0219 times 1800 = 39.35 cu ft.
3. You can also do the same transfill in reverse. Start with a full AL 80 and transfill into the empty tank. The caution here is to make sure you do not exceed the service pressure - if the unknown tank is significantly smaller than about 45 cu ft, you may have to shut the valve to stop the transfill at 1800 psi.
The transfill method is not 100% accurate due to the volume in the transfill hose, but it will be quite close.
