For the past few years I have dabbled in research as to who created the E135 set, a topic I had not seen discussed much. I am now confident in stating that it was our old friend Felix Mendelsohn, who followed his m101 offerings in 1916 with the cards depicted on Collins-McCarthy, Boston Store and other advertising backs the following year. While there are other indicators that pointed to Mendelsohn, the most important came to me yesterday from an ebay seller who was kind enough to provide a picture I had been seeking for a long time and once asked about in this forum.
http://www.net54baseball.com/showthr...hlight=origins
Here is the March 15, 1917 ad that Mendelsohn placed in The Sporting News, alongside an enlargement of the player depicted.

This no doubt shows that Mendelsohn’s proposed cards were designed the same as e135s. The fake horizon and washed out background elements were of course used in many 1920-1922 sets, but basically would have been new back in 1917.
Mendelsohn followed the March 15 TSN ad with another two months later on May 24, 1917. The only copy I have seen of that ad appears to show the same player photo, although it is smaller and the quality is horrible. It offers youngsters a baseball glove if they buy a set and sell four others, so it appears the cards were not a prototype (as I once thought). Instead, these were blank-backed cards of what we commonly call e135. They perhaps should more properly be called true m101-6s, and an argument can be made that they are instead the “real” m101-5s, since the prior year’s set now known by that name was never offered in the Sporting News. I will have more to post about these cards later, but wanted to get this out now.
[credit to ebay seller bensbigdad6tos for the photo assist]