B. -

you got it right.

wash your tuber, put it in a zip baggie with a barely moist vermiculite or sphagnum moss - and put it somewhere where it can get light, but not direct light - if it won't revive in 6 months - that's it. Just to be sure keep it for 6 more months and then chuck it.

Since the rest of your treasured Sinningias speciosas have the same age tubers - restart them from a cutting - just in case - you will have a young plant to replace an oldie.

The easiest way - is to take a tip cutting - old tubers usually produce more than one stem - you take one off and root it in a moist soil under the baggie. You can do it from a leaf as well.

Irina