The case almost certainly a Waltham case and near 18K gold. The mark is not a hallmark but a brand grade. Most US gold cases were at least 1/2K under that the mark stated.
If you mean by original made by Waltham watch case company. IF you mean original to the watch, you need ot check the movement side for case screw marks. If there are extras not where the case screws are now the movement is not original to the case.
Here in Brazil is very common watches in solid gold with the bow gold filled.
IWC, Vacheron, etc..
American watches with 18k solid gold is very hard to find in Brazil, this is the first that I found and I was think that the bow was original due the lots Europeans with the gold filled bows.