Your guess is as good as mine. Given the ballpark numbers for Robison '11 (6-15-2-1) and for St. Louis '20 (15-7-4-1) are reasonably similar for bp hr, it could well be that it corresponds to the 2 months of baseball played at Robison Field prior to the Cardinals move to Sportsman's. (Early dimensions were huge! Dimensions: Left field: 470 ft. (1893), 380 ft. (1909); shoot the chutes at the left field foul pole and in right center: 625 ft. (1896); deepest left center: 520 ft. (1893), 400 ft. (1909); center field: 500 ft. (1893), 435 ft. (1909); right center: 330 ft. (1893), 320 ft. (1909); right field: 290 ft. (1893); backstop: 120 ft. (1893); foul territory: huge.)
In any event, it's a good excuse to look at some images of Robison Field. According to one of the sites, there was an adjoining amusement park, a beer garden, a race track in the outfield, a shoot the chutes water ride with an artificial lake with ice skating in the winter. Now that's a ballpark.
http://www.thereusedtobeaballpark.com/product/robison-field/http://www.projectballpark.org/history/nl/robison.htmlhttps://www.flickr.com/photos/mohistory/4496980483/sizes/l/in/photostream/http://www.digitalballparks.com/NL/Robison1.html (be sure and click next picture for the ten photos)
Bill