Very rare Trimline phone with a line switch (explained below).
On July 10, 2012, I purchased a beige one of these I found on eBay for $49.99. It was in mint condition and was still in its original Western Electric box. After opening it up I was able to diagnose the wiring. Turns out it was a special phone intended for up scale fancy apartments or offices. I gather from Paul's website that this modification may have only been found in the Washington, DC area of the C&P telephone companies.
The original phone was wired using a 10 conductor (5 pair) line cord. The W/S pair supplied power to the handset lamp, the W/BR pair was not used.
In the normal position of the line switch (labled "Line" on my newer phone), the phone operates like a regular 1-line Trimline with the line coming in through the W/BL pair. In the other position (labeled "Secretarial") the incoming line is redirected to an operator's station (assumedly somewhere in the same building) via the W/O pair, so the operator can answer your calls and take messages while the rich apartment owner is out walking her French poodle and showing off her full length white mink coat. (Picture the rich sophisticate apartment dweller out of a 1960's movie)
When you got home you picked up the phone which buzzed the operator who would answer on a local intercom circuit via a 3rd W/G pair. Once you got your messages, you turn the knob back and your phone was normal again. If you forgot to turn the knob back, and a phone call comes in, the phone will ring but if you picked it up you only get the operator. However the operator will still see your incoming call and be able to answer it, or she can suggest you switch the key back to answer it yourself.
Very interesting wiring of a system that was made obsolete with the development of the ESS (Electronic Switching Systems) that came out in the early 1970's.
A schematic of the correct wiring on the base can be found at the bottom of this page. I'll try to get some photos of the inside of the newer phone next month. It's now packed away in the back of my truck and difficult to get to.
Rewiring project. My wiring on this phone now allows it to work on my 1A2 key system with A1 control. I used an 8 conductor network cable to simulate the look of the original gray style Western Electric cords.
I had to replace the original pinch mounted handset cord connector due to damage to several of the contacts. Not having a riveting gun I had to use nuts and bolts.
A closeup of the key switch wired my way rather than the historically correct way.
The underside. Note that the handset cord is not a WE cord but was purchased on eBay. At least the color matches.
Schematic of the AD1 Mod E2110 base as it was originally wired