Wein cells are zinc-air, but they're manufactured with fewer air holes, so they last longer in low-current applications (like light metering) before they dry out and quit working. The 675 are made for hearing aids, where they'll exhaust in a few days. Their lifetime in devices like the light meter in a camera is limited by how long it takes the electrolyte to dry out. If you can carefully seal two of the four holes with something like nail polish when you first unseal the cell, you'll extend the life, but in general, they're good for a few weeks to a couple months in our application.
The other difference with a Wein cell is many of them (PX625 replacement, for instance) are sold with an adapter washer to give them the same shape as an original mercury cell and ensure they can only be installed in the correct polarity. Some folks keep the washer from a Wein cell and transfer it to 675 (for economy) or use an O-ring to ensure the 675 stays centered in the battery compartment.
In my FT-2, the 675 cells just drops in; I'll put an S76 in there next time I buy one, and have a meter battery that will last a year or more. Some FT-N have been converted to use the S76 cells -- unless you're willing to take the bottom plate off and look for a zener diode, you could test with an S76 against a 675 to see if the meter reads the same across a reasonable range.