Wow, this is a bit of a stumper. I think there's too much left of the barrel for it to be the pen portion of a combo. The button filler mechanism is turned with the barrel as one piece, as previously noted. The chasing has me stumped too, as Parker really wasn't doing chasing in the plastic era. I guess the botton filling portion could have broken when the button was being pulled out, with someone doing some home pensmiting by cutting it out with a lathe, but I don't get what the "fix" would be. Sorry, I'm not being much help here.
Hi John and Brian.
Neat pen. Staying away for the moment from "what is it?", I'll touch on couple peripheral issues.
John, I could be wrong, but I believe that even USA-Parker did make documented (at least "known") chased plastic pens. Some of the last of the low line flat-tops, DQ and such, with longitudinal, raised lines (still considered a form of chasing), were made from plastic. At least I believe so. I probably need to do friction test on couple I own. These would be non-streamlined pens from just before Parker went streamlined, which occurred around 1929
But, I'd thought USA-Parker did not do streamlined chased plastic pens and probably did not do chased non-streamlined
plastic pens in the pattern Brian showed above. It seems Canada did make straamlined pens with this sort of chasing, based both on the Combo I recently described and on a related parker-Canada streamlined "no name" thrift-ish pen that resembles the pen we know from USA catalogues as the "raven black and gold" item. It has the same generally pre-streamlined-era chasing as your pen.
Your pen does resemble the Canadian "thrift" pen 1929+ styling (based on USA catalogues) and is the first I've seen in plastic with chasing in a USA-made streamline pen. Agree with John barrel seems long for being part of a Combo, though I'll need to confirm with my Combo. And, of course, your pen lacks the Combo top piece.
Will have to think on the back end. George has posted before about an anomalous USA_made Lucky Curve pneumatic filler pen. I recall we weren't wholly sure it was original, but were far from sure it was not. The back end seems famliar
Here is George's thread about the pneumatic-fill (Chilton-esque) slightly earlier-than-your-pen Parker Lucky Curve
Parker Lucky Curve with Pneumatic Filler
Here again is my recently found (only one known?) Parker Combo in chased (plastic) finish, this one from Canada. Again, I've not seen any USA made streamlined plastic Parker with this chasing pattern (prior to your pen). I have a "Thrift"-ish Parker Canada chased plastic pen of this sort, but it is buried. Will have to hunt for it.
regards
david