I doubt anyone left alive
really knows what the markings on the bottom of the barrels of Remington doubles mean. Sometimes, as in the case of this gun the R might mean Remington Steel barrels and the E might mean ejectors.
X
K
E
A1 on a 1909 vintage KE-Grade 16-gauge --

We can guess that K is the grade, E for ejectors and it appears that X is common to 16-gauge guns. What does the A1 mean?
///
K
E
Y on a 1906 vintage KE-Grade 12-gauge --

Again K could be the grade and E mean ejectors, but what does the Y mean?
///
F
E
G
13 on a 1906 vintage 12-gauge FE-Grade Trap Gun --

What does the G mean? What does the 13 mean?
///
M on a 1906 vintage 12-gauge CEO-Grade --

What does the M mean? If the O on the gun below means Ordnance Steel barrels, why no O here? Why no E for ejectors?
X
O on a 1905 vintage 16-gauge DEO-Grade --

Again, the X seems to mean 16-gauge and the O could mean Ordnance Steel barrels. Why no E for ejectors?
P
A on a 1896 vintage 12-gauge AE-grade --

A could be the grade, but why no E for the ejectors? What does the P mean?
B
J
E on a 1895 vintage 12-gauge BE-Grade --

B could be the grade, E could be the ejectors and the J could be the barrel material as the Remington Damascus salesman's sample lists these barrels as Chain J.
What does /// mean?