You are viewing a NOS (new-old-stock) 7-liter MB spare wheel mounted fuel canister. These are NLA (no longer available). The canister itself will fit multiple chassis (please check against your particular application). Canister has MB part no. B 6 7 58 0038.
Included (won't separate) with the item is an unopened bag of hardware for installation on a the S123 or S124 (hardware p/n B6 7 58 0033).
According to hardware instructions, it is compatible with:
1. S123 with steel/light alloy rims
2. S124 with steel/light alloy rims
You can absolutely purchase the unit but not use the wagon mounting hardware, and install it on your other classic MB instead. I tested this on my W123 300D with bundt wheels, but the bundt wheel original spare does not allow the unit to fit in the spare tire well due to a structural spare tire mount.
I last opened the unit earlier this year, and kept it in storage since. When I opened it again for the photographs below, the lid had a swoosh indicating an airtight seal since the last time I opened it up.
Inside is the red/orange coating mandated by TUV, untouched, dry and with no fuel smell. This is an unused unit.
There are some slight rust spots on the metal spout, but these appear to be cosmetic only.