Here's what it says about this product in the company's literature. The underlining is mine.
The amazing Silver Trap Window ST-WD and ST-WDC
(compact) metallic replacement cartridge system is the
only metallic replacement system you will ever need for
silver recovery from Medical Imaging/B&W Fixer.
Window’s unique ultra-compact, clear, modular design
can meet any installation requirement. Windows are
engineered around a patented, time proven iron-matrix
screen core, which reduces replacement frequency. This
high-surface area core is the longest lasting, most
efficient, sacrificial catalyst material ever developed.
It's basically packed with a roll of iron screen (sometimes copper plated), instead of the more common steel wool packing. As it says, the iron is sacrificial. In other words, as the silver cements onto the iron, the iron dissolves. Therefore, the more iron remaining, the less silver is in there. If all the iron is gone, you could have up to 60 oz of silver. In practice, though, I doubt if 60 oz has ever been recovered from one of these. The maximum capacity is probably just a mathematical calculation based on the weight of iron used. How much iron is left? Can you get an approximate weight?
I've processed a lot of these but never chemically, since that would be quite difficult and would be a mess. There are 3 things to deal with: (1) the remaining iron (2) the visible silver, which is usually a gray color (3) the black sludge probably present, which contains silver sulfide. Not an easy combination and the more iron you have, the more difficult it would be.
The standard way (in general) is to first remove any plastic, dry the cores and sludge, and then melt the cores and sludge in a gas-fired crucible furnace fluxing with borax/soda ash, and using a few lengths of rebar to react with any sulfur present in the black silver bearing sludge. If there is a lot of iron, the rebar is probably not needed. Also, a lot of iron will require a lot of flux. And, the more flux, the more silver will be trapped in it, usually as BBs.
I don't want to discourage you, but these are one of the most difficult-to-refine items on the planet, unless you have the right melting equipment and know what you're doing. With some exceptions, about the only refiners doing it are those that specialize in it and do it in large volume, probably in reverberatory furnaces.