The late Austin Hogan did a very good summary on this very topic. I believe it was published in the Parker Pages a number of years ago. There may even be some old posts on the board relating to it.
And yes, every 12 gauge Parker I have personally examined that was built in the approximate time frame you listed, have all measured around .750.
10 gauge guns of that vintage also seem to follow that configuration. Can not speak to other gauges from personal experience though.
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“Nitro cracking in woodcock cover has something about it as respectable as a village church bell.”~Gordon Macquarrie
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