That jacket is seriously cool, I love it! I would say definitely 70s, but I don't know the exact year. It has the wider lapel and notched collar. You see a lot of notched collars where the top portion juts out farther than the main lapel in the 40s also, but the proportions for 40s are much narrower.
Re: junior sizes..
Junior sizes can be bigger. Its the cut that is different, just like today''s Juniors department. Juniors sizes go up to 15 and Junior Plus sizes go up to 21 and even up to 25 in some stores. In the 50s, it probably wasn't true..but there were "half sizes" that worked better for petities and juniors as you went up the scale versus just "junior miss" I know size 8-10 were considered junior miss back then and they were about a 30-33 bust.
According to JCPenneys.com, these are dimensions of the wearer not the flat measurements of the clothing (modern measures)
A misses size 6 is a 34 1/2-35 bust, 26 1/2-27 waist and 37-37 1/2
A petite 6 is 34-34 1/2 bust, 26, 26.5 waist, and 36, 36.5 hip
A juniors 5 is 33-33.5/25-25.5/35.5-36
A juniors 7 is 34-34.5/26-26.5/36.5/37
a 9 is 35-35½/27-27½/37½-38
The bust/hip/waist in juniors is more comparable to a petite proportion, but since the junior line is made for women 5 3.5" - 5 7.5", the rise is longer and it creates a slimmer cut, where a misses size assumes you have a fuller hip compared to waist. It makes sense to me because it corresponds right in line with the "half sizes" that were available back then. But I will point out that some of the half sizes were apropriate in length for the taller lady and some were not. I have two half size dresses that the shoulder to waist ratio is small, but based on the skirt length it would be entirely aprorpiate for a taller lady than I with a smaller torso(as i am short) , but the other one isn't.
By the way, The May company owns Filene's Department stores currently and I forget what the other nameplates are.