On the topic of the earliest Parker SBTs, this gun does not have the finger groove FE
Fred Gilbert took delivery of a Parker SBT SN 180116 in 1917. In 1919, at age 54 he made a straight run of 589 with it.
Other examples
Outers' Recreation, November, 1919
https://books.google.com/books?id=l3...J&pg=PA383&lpg
October 1919
Forest & Stream illustration without the finger groove
https://books.google.com/books?id=F9...J&pg=PA568&lpg
Was the finger groove FE the standard? When was it discontinued?