19and41, you read my mind. I was thinking about making a post on that same episode. That's a great Perry Mason episode and it's thought to be the pilot show of the series--the very first episode filmed. The Case of the Moth-Eaten Mink, filmed in October, 1956. I think you're correct about there being product placement because no other episode I can recall has so many new Western Electric phones.
Besides the Cordless PBX, they have lots of soft plastic phones in that show too. We see color 500s in Perry's and Paul's bedrooms and on Paul's desk there are three different color 500 sets. The one they use is either brown or dark gray. It looks black, but it has a clear fingerwheel. Also on Della's desk is a dark beige 500 with gray cords. In one scene, the secretary in Paul's office uses the PBX to patch through a call on an outside line to Perry Mason at home. That may have been the reason for the PBX.
I finally found my Season 1 DVDs and here are some screen shots...
1. Paul Drake's office. Here is the 507B PBX. Paul's secretary is using it, along with what looks like a red 500 set.
2. A different view of the same setup.
3. Paul Drake at his desk with three soft plastic 500s. Brown (possibly), dark beige and red.
4. Della Street at her desk, on a dark beige 500 with gray cords.
5. Perry at home in bed next to a red 500 set.
6. Paul Drake at home in bed next to a red 500 set. This is a really nice shot of the side of the phone. Also, he and Perry have similar headboards, but Perry's looks to be velvet and Paul's is a plainer fabric.
