Hold on there.. you say the rev back is missing one pin?.. only one silver tit? You have an RB not a a Pro back then.

Unless you mean the pin is not firing at all?

It may be bent. Take the rev back off n push the pin out manually n inspect it? If it is sticking.. a tiny drop of WD will losen it up. If it is bent.. use a flat jaw pliers to work it flat n straight again.

OH also if you can't find a solution other than taking your film back off each time.. jsut throw the lever to multi exposure instead of using the doubnle exposure secuirty feature.. the lever is under the cocking lever on the back.. red dot there?