Dating convention of vintage and antique items is pretty simple and follows standard dating rules. But it is often widely misunderstood. These are the three main dating terms we use on a daily basis.
Circa — this means approximately and in describing the age it means +/-10 years. Therefore, the date circa 1920 means between 1910 and 1930. In most cases we use circa when we think the actual date is just a few years around the stated date. Statements like circa 1920s or circa 19th century are not correct, we do not use them.
A decade in the dating, for example 1930s — this means the age we refer to is between 1930-1939. It is very similar to circa but a bit narrower (by 10 years) and gives the age description a slightly different focus. This logic can also apply to a century, so you can say 1800s, meaning 19th century (1800-1899) — this is of course a very broad dating and we use it rarely.
A century in the dating, for example 19th century — technically, this means the years between 1800 to 1899. This is a very broad dating and we use it only when we are unable to put a more precise date on the item. Yes, such things happen, especially when the stylistic features of an item are very generic, or when an item had been in production for a very long time. Where possible, we try to narrow such dating, at least by saying early 19th century, mid 19th century or late 19th century and so on.