Hi Sheila, I agree that this is a modern set, probably from the '90s. I can't see '40s or '50s influence, it looks very modern to me but perhaps hand made by a skilled seamstress.
Joji, serging (what we call overlocking in Australia) is an old technique that's seen on menswear from the early 20th century, and ladies lingerie from about the '20s. I've seen it in dressing gowns and house dresses from the '30s onwards and lots of dresses in the '40s but there are differences in serging techniques in different eras according to the machines available. I don't think the home sewer had access to the technology until the early '80s (maybe a little earlier) and I think that's why there is this myth that only modern clothes have it. Factories used overlockers/sergers routinely since WW2 and mass production really took off, but you don't see it in the top quality garments because they were still hand finished. As well, overlocking/serging can be added to a garment during a modern update or alteration - it's easy to tell if this has happened because the stitches won't go right into the corners but stop a little short, veering off.
Re: zippers, they're often replaced so it's helpful to look closely at the stitches for signs that you're not looking at the original fastening - eg, different colour thread, small holes or marks where the zipper was originally and had been unpicked. Maybe the zipper is sewn on top of the lining rather thn inside, or inside the facing. There are many things to keep in mind with dating vintage because updates and repairs are not uncommon.
