I was just looking at a 20-gauge for sale on GI that includes the original hang-tag and has a letter. The letter states "RH full and LH full."
The hang-tag states the gun was targeted with 2 1/2 inch 20-gauge shells with 2 1/4 drams of bulk smokeless powder and 7/8 ounce of #8 Tatham Chilled Shot. The tag states R.H. Bbl. targets 100 pellets, L.H. Bbl. targets 150 pellets. According to the charts I've found, Tatham #8s run 399 per ounce which would be 349 in 7/8 ounce. 100/349 = 28.7% and 150/349 = 43% essentially extremely open.
https://www.gunsinternational.com/gu...n_id=101458959