OK, it's the FIRST generation cardboard container...
Hercules, to the best of my knowledge, began using them in the early to mid 1960s before going to the round can with the plastic pull spout in the 1970s.
While the inflation analysis is interesting, I really think it's of dubious value in setting the date simply because as a commodity product, whomever originally sold that powder could put just about any price he wanted on it, high OR low.
Even today there can be a tremendous variance on the price you'll pay for a can of powder.
A few years ago a friend of mine paid over $25 for a pound of powder at a shop here in the DC metro area.
Had he been able to wait for the gunshow that was coming a few weeks later (where he did buy an 8 pound jug), that same pound of powder would have cost him $16 and change. But he needed the powder for a match, and it was a powder that I didn't use, so I couldn't give him any to get him by.
It could also be that the powder was a "forgotten can" that the store owner simply slapped a low price on just to get rid of it.
Any of a hundred plausible scenarios, each giving a different potential date...
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.