yes the 51715 WIX is the correct number. Put one on my 4940 yesterday. I looked at Baldwin's website and I think the problem with the Montana is the metric sizes. My 4940 has a 40mm and I don't think Baldwin is making anything for that size.
also look around before you buy one. Oreilly's wanted $50 for the hyro filter. found mine online for $23 plus shipping and my local Oreilly's match the price. worth a shot to try yours