First, NO E-models have appeared below 54000 with only about 20 in existing collections (as of 2007) with 2 of these being in the Smithsonian. As a result, the previously accepted range of 50000 to 55000 is questionable. It would seem more likely that the true range was from 54000 thru the low 60000s.
Secondly, I have seen only one "F" machine with a SN under 100,000 and it is clearly special (see below) and have concluded that all standard F-models started at 100,000.
McCarthy goes on to report a project to convert some of these slow selling Es to Fs, probably around 1915. Indeed, a machine exists that appears to be a normal F-model in most all respects except having an "F" roughly stamped in front of its 55376 serial number and a slightly different front mechanism. Such conversions could have been an effort to recover some of the investment in this apparantly abandoned model.
The most strikingly different feature of the E-model was the keytop/keystem design as evidenced by this photo and the patent drawing below. All of this was to aid in operator error detection so the "Controlled Key" feature could be activated. While its a bit hard to see how all this works from the photo, the patent drawings are quite explicit.
Note this clip from the Jan 24, 1911 patent (982,416) showing keystems with attached metal side plates and spring encased within the stem! In production, the keytops were oblong to allow room for those side plates designed to detect "fat fingering" errors (this was brot to my attention by Kevin and Justin Odhner, caretakers of one of these rare machines).
A reading of this quite brief patent reveals Felt's devotion to providing operators with maximum error detection and correction capabilities. He states, in part, "The releasing devices are so located that they are depressed necessarily when the finger presses properly on the center of the key, but not when pressed on its margin."
This model carried a white "Controlled Key" button rather than the more common red one that was to grace all future models. These buttons however, were easily replaced and, therefore cant be considered a definative model indicator.