The OED (229) says a finger stall is a protective covering for an injured finger. Several other sources (56, 171, 181, 302, and 320) support this. Halton (147) says it is a cue bridge, i.e., one of those scalloped fittings on the end of a stick for guiding the cue when you are too portly to bend over the table. Knight (177) endorses this. The fact remains, however, that none of my English friends professes to having heard the term used in that sense. MacPhail (194) has uncovered a reference to a tailor-made device that Gilbert purportedly used on the fingers of his left hand to act as a cue guide. MacPhail believes that is what Gilbert meant by a finger stall. Be that as it may, I have yet to see any evidence that Gilbert or anyone else ever called the device by that name. My vote is with the OED.