My Dad had this for as long as I can remember. The last time he went hunting, it was never fired.. No deer, no joy. Probably sometime in the early 60's. The operation is smooth and clean. Stored in a gun box in the attic.
If you suck, go back to the range.
Can't help you with the value question, but I think the stock's been sanded to the point that a fore-stock graft wouldn't be worthwhile. If it were mine I'd replace the stock, and I'm sure there are members who can direct you to a source. The 42 receiver code indicates Mauser manufacture as best I recall. I've never seen markings such as that 3Ru261. Haven't a clue what they stand for or how they might affect value.
Good thing that none of the metal appears to have been altered. Mauser purists are likely gathering with torches and pitchforks to visit my farm, but I agree with nmckenzie - I'd get another full military stock and put it back together. I'd do some research and find the correct pattern (laminate, etc) stock and bands.
Value? As-is I'd say about $150 - $175; With grafted on fore arm - same - not going to add any value; With repacement stock and bands I'd figure about $250 to $350 at a gun show, maybe slightly more if you find the right buyer. Mauser collectors want rifles with un-touched original stocks, markings and all original matching numbers on ALL parts.
Your stock is way too far gone (cut/sanded/drilled for swivels,etc) to restore. I'm sure your dad was happy with the rifle - enjoy it!