Rather than fabricating plugs from walnut stock (or from the grip itself) you may want to consider simplifying the repair by searching online for a walnut dowel, then enlarging the holes as needed to the diameter of the dowel. A quick search turned up a number of sources, but I didn't go into great detail looking at diameters that are available. I'd probably opt for 1/4" myself (assuming that size is available). Then I'd place the grips in my drill press vice and carefully enlarge the holes to a diameter of 1/4" using a sharp drill, at a depth of about 1/4" or so. Then cut the dowel to a tad over that length, glue in the hole as a "plug", and once it's dry, sand flush. The end grain of the dowel should be a fairly close match to the end grain of the grips.

Good luck,
Lou

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NRA Certified Instructor & RSO