Dick, you haven't answered and maybe you won't.I make a preliminary summing-up anyway.You see - the tapping exercise as described can not be expected to improve the "strength" 1) (momentary maximal power) at all.It may possibly improve "strength" 2) (endurance) a bit but hardly any better than playing the instrument itself. The same
regarding 3) coordination.
The Gripmaster may possibly be efficient for improving 1) up to its highest grade which seems to be 3,5kg resistance - not very much.For improving 2) it likely may be more useful.I have found no objective evaluation of its usefulness neither in rehabilitation, nor in sports or musical activities.
There is no reason to believe that this kind of "fingerstrenthening" exercises have any significant positive effect for concertinaplaying that you can not achieve by systematic playing of the instrument so I mainly see it as wasted time unless the primary aim is to relieve your neighbour from listening to frantic scale exercises...