Your Western field model 550ED was made by Mossberg for Montgomery Wards.The Mossberg model 500E and your shotgun are one and the same.The Mossberg model 500E shotgun has avalue of between 85-170 dollars,depending on overall condition and a good bore.
No published sn data. 50's or so
Gun shows, gun shops, pawn shops, estate sales, garage sales, book stores, on line auctions.
50-200 USD
Numrich has a cross reference list (http://www.e-gunparts.com/crossref.asp). It seems that your shotgun is the same as a Mossberg 500E. I have the same gun and ordered parts from them. Here is the site that has a parts list. http://www.e-gunparts.com/productschem.asp?chrMasterModel=0840z500