Troy,
Let's say at least up until WWII, for Glascock on the lids & bottoms of their 3 legged Spider, both with gate marks. One family member went to work for foundry in 1937, got special permission to be late to Boot Camp in 1943, due to helping foundry put in new cupilo(sp), came back in 1945 worked for foundry until Union strike of 1946. This family member told me they were still making this spider during this period and he gave me a never blacked #9 lid. I never discussed gate marks with this family member, but am just going by the spiders that they made.
I to have heard the gate marks are suppose to be old, but I also think it depends on the foundry and how they operated. A lot of other Glascock products from this time period don't have gate marks, so who knows. Personally I've always thought the large production items(stoves), probably didn't have gates marks in the later years, but the smaller items, made off in some corner of the foundry, probably were made the old fashion way.
This same family member told me that Skillets & these Spiders were made of left over iron at the end of the day.
Of course this was Glascock, I'm sure Griswold & Wagner, were like Lodge today, doing modern things for their time period.
Come see us when you can.
Take Care,
Nollie