https://www.ebay.com/sch/Fenton-Art-Glass-/2719/i.html?_nkw=fenton+silver+crest&rt=nc&LH_Sold=1&LH_Complete=1
Most likely these were sold as “seconds” !! Most factories sold those that are perfectly usable but not top quality !! Fenton had a marking system for seconds but did not start it until the 1990`s !! Yours were made before that !! If someone wants these plates they may not care if they are imperfect !! I personally think they make them unique,, but that is just me !! Perfection is not a word I use often !! The hard core Fenton collectors may be different but some look for these types !! Above are the current sales from ebay,, may give you an idea as to what they will bring !! Fenton is not very expensive !! The asking prices are not what they actually sell for !!
No, there isn’t any mark on the back. I will post a picture tomorrow of one of them. Thanks