What I mean is, you'll need an old model trigger group-trigger guard assembly (both are the same) to fit it I think, but I would try one from a 500A, if you don't have the old one, to make sure it will work. Numrich might have it, but they do not show it, for the 500A. The only place you might find the entire group is on eBay, a gun auction, or someplace that has a bunch of older Mossberg parts.
My guess is that your Western Field will have that older style trigger in the guard. I wouldn't be able to really tell, unless I looked inside the frame, to see how the safety was set up.