Will be date stamped for month/year check here: http://remingtonsociety.com/rsa/questions/barrelcodes
They only made them from 1936-1939, that's probably a first or second year production.
1920
That is not a serial no of a Remington 1911A1 they stopped in 1945 with 2465139.
I checked www.oldguns.net and they show the manufacture date of Remington 1903A3 serial no. 3851292 as 1942.
RW isn't part of the serial no, it stands for Remington Works 1915
1912
1920
1924
1912
1912
1923
1914
1911