All sources I find say the British did not start putting dates on web gear and slings until the 30s. This is an impressive fake then...well aged or an older sling with a newer fake stamp.
Are you serious? There are tons of fakes and réplicas of these slings. WWI originals, when found, bring a lot of money.That could even be a unit mark, applied locally for some reason. Usually fakes use well-known and rare makers to run up the price. Those slings are hardly pricey enough to justify making a bogus set of replicas.
Yes. Much like their use of "stoving" (black paint, basically) some of the use of rubber stamps and ink seems to have become promiscuous both by the actual U.K. and commonwealth "authorities" and by the fakers.That someone would go through the trouble to mark a real sling with a fake stamp only to sell it for less than the real sling would go for is less believable than that marking itself.
As we all know, every ink stamp ever made was always applied completely legibly for all of eternity.