Those magazine guides on the rear of the magazine are noted for separating, like you experienced. I had a Mossberg 195 when I was a teen back in the 60's, and kept the local sporting goods dealer in business by buying another magazine every time I had one pull apart. I had a buddy who was an Industrial Arts major when I was attending community college, and he brazed the guide to the back of the magazine; this permanently solved the problem.
Being the moron that I am, I sold that Mossberg 195 back in the mid-70's, but fast-forward to the mid-90's. The local hardware store had a used Mossberg 395 for sale that I promptly bought. I noted with interest that the previous owner also had the magazine guide brazed to the magazine!
Enjoy your WesternField 185; they are fun guns!