There were notices in spring issues of Breeder & Sportsman regarding a Parker Bros. publication
https://archive.org/details/breedersportsma651914sanf/page/n180/mode/1up?view=theater

which then appeared in the October 3, 1914 issue
https://archive.org/details/breedersportsma651914sanf/page/n225/mode/1up?view=theater

It appears to be an abbreviated version of Parker Brothers' “The Small Bore Shotgun” which was published c. 1915
http://parkerguns.org/pages/PDF%20Documents/Small%20Bore%20Shotgun.pdf

Edwin Hedderly had published his first of many articles promoting small bores (and Parkers) in Forest Stream, December 26, 1908 “Twenty Bores for Duck Shooting”
https://books.google.com/books?id=ejQevDPMUIYC&pg=PA1018&vq

Nov. 13, 1909, “Small-bore Guns”
https://books.google.com/books?id=l0kcAQAAMAAJ&pg=PA790&lpg

A few more
Arms & The Man 1915, “Loading Small-Bores”
https://books.google.com/books?id=BX07AQAAMAAJ&pg=PA158&lpg

Recreation 1917, “Handloading for Small-Bore Guns”
https://books.google.com/books?id=4uVQAAAAYAAJ&pg=PA97&lpg

Outdoor Recreation 1919, “Loading Small-Bores”
https://books.google.com/books?id=BX07AQAAMAAJ&pg=PA158&lpg

Field & Stream, Nov. 1921, “Sixteen” – Or “Twenty” Which
https://books.google.com/books?id=UPtAAQAAMAAJ&pg=PA704&lpg

and

T.H. Grant, Forest & Stream, December 1915
https://books.google.com/books?id=lRMcAQAAMAAJ&pg=PA711&lpg

Forest & Stream, Nov. 1918 “Keener sport with the 20 gauge gun”
https://books.google.com/books?id=b91JAAAAYAAJ&pg=PA663&lpg

Outers’ Recreation, July 1921 “Seventy Years Experience With 20 Gauge Guns”
https://books.google.com/books?id=hns7AQAAMAAJ&pg=PA76&lpg

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