Jim,
Somebody has tried a deep cleaning to degrease maybe using oven cleaner (which contains lye, a known wood fiber eater). Or maybe a too aggressive dish washer cycle.
My WAG is a caustic cleaner was used followed by a water blasting. No Walnut should have a open grain pattern like this. If it did, they might as well have used oak for stocks.
After a blasting like that, they had no choice but to sand away the splinters.
I pulled stocks from laying in piles outside for years that showed sun fading and mildew but still had tight grain.
Remember FRANKEN STOCK ??