I'd Tell Him....
I'll fax off that 40 bucks right away. I don't know that I've ever seen a sling marked with manufacture date on it, or even packaging with the year of manufacture on it for that matter, they list the item number and contract number only usually. The ammo is dated usually, and the "born-on" year of the firearm is available by checking the serial number. I tried to talk my local gun shop into including one of the slings off a few other carbines they had when I bought my Rockola, it didn't work though. Sadly, I know they were actual correct slings and oilers, as I knew the guy who consigned them for sale there and he had got them immediately after WW-II someway or another from the military, he never told me how.
I thought I had maybe paid a bit too much for mine until today when I saw my CMP catalog, and Rockolas were listed for $40.00 more than I paid for mine just a few weeks ago.