I have over a dozen of these bottles. None of them have traditional date codes, pat numbers, nor anything on the bottom surface. They all have a 1 digit number then a varying symbol and a 2nd single digit number.
Many people are under the impression that there is all kinds of info available for mold markings. It is unfortunately not the case.
Your bottles are at oldest 1980s or 90s. Not sure you are going to get any more detail than that