The post about "too much misinformation" in railfan press, circles and websites is right on. If you're going to publish a book, or put up a website, do your homework.
My brother and I are here looking through his manuals on this subject. It's true that a pneumatic throttle arrangement was available for years on EMD switchers which did not have multiple unit control. Manual 252B for the Model 567B engine, on pages 1103/1104, displays a Woodward governor with an internal diaphragm arrangement, very similar to that used on Baldwin-Lima-Hamilton units later on (modified Woodward PG; BLH manual DE-111A, Feb 1955.) The info we have on the EMD units notes that the pulling of the throttle away from idle actuates a cam switch to close the shunt and battery field contactors, applying power to the traction motors (again, more similar to the Westinghouse CE-100 controller than to the D-1 which required an external air-actuated throttle switch.) Note, of course, that Baldwin units frequently used air throttles with multiple unit controls; it isn't ruled out theoretically overall, just on EMD's. (Oh.. and don't forget F-M units with air throttles.)
The BL2 did, of course, have an electric throttle arrangement which could be used with other EMD units with MU. The BL2 was essentially like the F-3 electrically; however, the throttle lever itself in the BL2 does not have an escapement latch, which allows rapid opening.
(The previous comes from the 'Supplement to Manual 2308A for use with model BL-2 locomotives, which is a 5/48 manual without the 10/48 updates.) The BL-2 had automatic transition without the the transition forstalling feature found on F-3 units. BL-2 also had a Load Regulator Control Switch on the control stand, which had "Road" and "Switching" positions. The Road position allowed modified maximum field start, as in the contemporary F-3. In Switching, a shunt resistance is inserted, which bypasses the load regulator, allowing more rapid starting. (This is very similar conceptually to the 'teaser starting' employed on the GP-7 later, and was done with a single 7.5 ohm resistor.)
One thing of interest. Where, exactly, does the "BL-1" label come from? As we've looked through all of this (which isn't exactly new to us) we find NO use of the BL-1 designation in any of the original EMD materials. We find two designations: BL, and BL-2. This is not, in fact, surprising. After all, we all know that the FT was followed by the F-2 (yes, yes, we know about the F-3 being tried first and all that.) My point is that there was no F-1. (There was also no F-5, either, and nowhere in any official material will you EVER, EVER find this; it's a railfan creation, and that's it.)
If you look only at original manufacturer materials, you find two types. BL, which is the original prototype with air throttle, and BL-2, which includes production units with electric throttle, whether or not they had MU. The supplement in 2308A is very clear in the diagrams given for single control/no MU and dual control/MU that both are model BL-2. As a specific example, if one looks at manual 252C, 12/56, page 1119 gives a listing of settings for governors to be used on 567C engines when backfitted and operated at original unit horsepower. Right there in that table is a listing for "BL." No number. (In fact, this listing gives a replacement pneumatic governor.)
That's what we have for now; if anyone has further questions which can be answered from these manuals, let us know.
Will & Dave Davis