Remington Model 1900s are a simplified, cheaper, version of the Model 1894, built on the same patents -- No. 528,507 and No. 528,508 both granted Oct. 30, 1894. The Model 1900s were all K-Grades, with E added to the designation if the gun had ejectors and D if it had Damascus barrels -- K-, KE-, KD-, or KED-Grades. The K- and KE-Grades had Remington Steel barrels. The Model 1900s had a snap-on/off forearm and their serial numbers were in the 300,000 range, often preceded with a stock letter Q.
You need to check out Charles G. Semmer's book "Remington Double Shotguns." It is available from the author 7885 Cyd Drive, Denver, CO 80221, for $60 plus $5 shipping and handling. It is invaluable if you are going to shoot, invest, collect or play in the Remington double gun field. Remington supplied a number of different pattern Damascus barrels on these old doubles. A picture of their salesman’s sample of the various styles of Damascus available is shown on page 275 of Semmer's book.
Remington Arms Co. stamped the actual pellet counts of their test patterns on the rear barrel lug of their Model 1889 hammer doubles and their Model 1894 and 1900 hammerless doubles. If the number is three digits, that is the count, if the number is two digits a leading 3 is implied. From surviving hang-tags we know the standard load they used to target 12-gauge guns was 1 1/4 ounces of #8 going 511 pellets to the load. My 12-gauge KE-Grade Model 1900 is stamped 33 on the left and 24 on the right. That would be 333/511 = 65% left and 324/511 = 64% right, or about improved modified in both barrels. The chokes measure .027" in both barrels of that gun.
I don't think anyone has really decoded those letters and hashmarks down the barrel tube bottoms.
