Yes it is.Is that an Army field jacket?
Thank youI don't know if this is helpful, but the RN 43031 is matched to a company called "GIBRALTAR FABRICS INC", which may be the same company as "Gibraltar Industries Inc." a company that did produce military apparel in the past. However, every Gibraltar Industries military piece I have seen says the name of the company, not the RN. I personally don't think I've ever seen true surplus items with RN numbers on them.
Anyway, don't really have anything solid to say, but if the jacket is a reproduction (which I wouldn't be surprised if it is), it is likely made by a company that has produced military garments in the past, so you could do worse.