Boy, would I have lost a bet on that gun. That gun is a model 99EG obviously.. with a factory installed model 99R buttstock last made 3 years earlier. That 3pt checkering pattern on the 99R buttstock ended around serial number 355,000, by late 1938/early 1939 they had just 2pt checkering. Though.. I have a rifle with it from 1945. Similar to yours, a special order non-standard model. The triangular checkering patterns on your forearm do look like the checkering that was added to the 99EG around serial number 387,xxx - but there was never a diamond checkering pattern on the bottom of the forearm that joined the 2 triangles.

I don't know if yours was a special custom order which would account for the side panel checkering, or just a 99EG assembled with a leftover 99R buttstock, or an early 99EG checkering prototype... the records are gone, so there will never be a way to be sure. So we know:
1) the buttstock/receiver/forearm are all factory.
2) the side panel checkering and bottom forearm checkering are unknown, I would guess aftermarket just because we haven't seen things like it before. But that's a judgement call.
3) studs and pistol grip cap will also be aftermarket.

It's a 1 in 10,000 gun that doesn't fit into a standard category. Never say never.

For reference, here's an early 99R with the 3pt checkering on the buttstock. Slightly different than yours, but not a lot. The 3pt checkering on 99R's ran from 1932 to 1935.



Here's a 1941 99EG with standard EG checkering.