Lookup date code on Remington Society of America here:
http://remingtonsociety.com/rsa/questions/barrelcodes
Go to the Remington Society of America and click the Manufactured link, it has the location and barrel codes for month/year of manufacture.
Serial 718943w
100-350 USD
http://remingtonsociety.com/rsa/questions/barrelcodes
See related link.
They have date code stamps on the barrel which can be looked up on the manufacture date link on the Remington Society of America. --- Update: It's important to note that the barrel on the shotgun MAY not be the barrel that was originally on the shotgun at purchase. Check the serial number with Remington.
The barrel date code is the primary method of dating a Remington. Of course, if the barrel is not the original barrel for the gun that complicates things. The Remington factory could probably date it by serial number. sales@countrygunsmith.net
Made by the Remigton Firearms Company.
Contact Remington Customer Service thru their website.
The W ahead of the serial number indicates that your Remington 870 wingmaster was made in 1972.
Check this table for the answer: http://www.remingtonsociety.com/rsa/questions/barrelcodes
It's date stamped see related links for info.