What do you mean by "How rare is this?" People used to craft at home, and knit patterns are readily available. If there is no tag, it could be homemade. If that is the case, you may have a better time discerning the era by looking through patterns, and doing a burn test to determine the fiber. I also usually segregate new-to-me knits to the freezer for a few weeks, just in case it's natural fiber (wool or silk) and anything has made a nest and laid eggs. After the freeze, I do a gentle hand cleaning -- often with older knits, more places need mending than are easily seen on first pass! It's disappointing to mend, wash, and then find more mending needs to be done.
The collar may be the key to determining this ones age - that is very unique! Sorry, I don't have the answer, but knowing the fiber will definitely help.