I can confirm from experience that the Wein Cell MRB400 works perfectly in an SP1000! In fact the meter gives remarkably accurate results, even using transparency film. After my first Wein Cell expired, I pushed the actual battery out of its black (rubber/plastic?) washer and replaced it with an identically sized 1.35 V zinc/air hearing aid battery (I can't remember the number) and that continues to give perfect readings several months on at a fraction of the original Wein Cell price.
I've read several reports that state that the Spotmatic series use a bridge circuit for the meter, making its accurate operation independent of battery voltage. (Interestingly, I've never seen anyone produce an actual circuit diagram to prove this, although I'd welcome the opportunity to see one.) To test this out, I've compared an original mercury battery and its Wein Cell equivalent (both 1.35 V) against a 1.5 V alkaline battery in my SP1000 and the latter causes the meter to consistently "over-read" (i.e. underexpose) by anything up to 2 stops.
So, a vote for zinc/air hearing aid batteries here with a little help from the holder from a Wein Cell or the voltage-reducing H-B adapter with silver oxide cells Casey mentioned above. Given how cheaply the hearing aid battery route gives accurate exposure meter readings and how easy it is to buy the batteries, I can't see any point in recalibrating an already properly set-up camera for alkaline or silver oxide cells.
