If it's an older model I do not know but the newer made pocket model seems to take the same size wrench as regular sized revolvers but some wrenches may be too thick to fit down into the area where the nipple is seated.
For example the universal nipple wrench, my favorite, won't fit, but a small cheapo (but good quality spring like steel I think) double sided nipple wrench with the sliding T handle in the middle I have had for ages that works on Remingtons fit's the slots of the newer pocket model fine on both ends. Each end is slightly different but not enough to make it non working.
The cheaper one has less metal around the outer perimeter (thinner) allowing a fit. I wish I remembered the brand name but I have long forgot and it's unmarked. I am glad it was handy.
Now I don't if the .31 calibers have the same size dimensions as the .36 pocket models as far as the nipples and recesses are concerned. My impression is there is no difference at all. Maybe someone else knows for sure.