OK, back when I first got into the public safety business in the late 60s, there was a widely used commercial incident report recording system that was attached to the law enforcement HQ switchboard. An officer could call in and quickly record his/her basic report, then later dial into the system and play back his recorded report as needed to complete the hard copy with a narrative. These systems had standard hardcopy report forms with blank fields that the writer used to record his/her initial report, then when played back he/she simply matched the fields.
PX was the report writing shorthand for the public phone system, PXO was shorthand for Public Phone Operator which meant the call came through the public phone operator, PBX was, as noted, Private Branch Exchange (actually a phone industry term for an internal phone system supplied by the phone company but limited to the confines of a large commercial or otherwise complex), and PXP for a private phone exchange, such as the systems sourced and installed privately in a large applications.
In other words, in the blank marked "How Received", you put PX if the call came in by phone and a reporting party was actually talked to, PXO if it came from the phone company operator (an anachronism) and the reporting party was talked to, PO if the call came in through the operator but no party was talked to, RX if it came in by radio, TTX if it was received on the Teletype (another anachronism), etc., etc. Over the years, PX became the generic for a phone call. It's what was used generically for years by the time I was hired.
