Bill Hudson forwarded his non-functioning Depotronic and Circuitron flasher boards for me to see why they did not work with his AF signal. (see
https://ogrforum.com/topic/af-760-crossing-flasher?page=1
After some probing around and measurements, I found the boards were functioning but just not with the AF signal. Turns out the fault is the this particular brand of LED bulbs themselves. The screw base bulbs are wired with their solder tip positive and brass shell negative. For AC applications polarity won't matter, but in case of AF signal it does.
Depotronics as far as I could determine has ceased business (their website URL leads to a Chinese page), so I have taken the liberty of sketching out the schematic shown below for reference of what follows. Because Circuitron is in business I can't show their schematic but the same no flash problem is exhibited when using this LED brand in the AF signal setup.
Bill rewired the AF signal as described in the OGR article making the signal frame and screw shells grounded, and each lamp bought out on individual wires. Now connecting the frame to positive supply Flash Return pin 4 orients the LED cathodes to the positive terminal, and FL1 and FL2 LED anodes to their respective NPN transistor collectors. The LEDs are thus reversed biased and cannot light!
Assuming that the AF sockets could be insulated and correctly wired to the respective terminals, a second problem crops up. Note R12 and R13 shunting their respective NPN transistors. With LEDs drawing little power, both simultaneously light through their resistors, including them as an additional series current limiting resistor, ignoring the transistors turning on/off. All that happens is LEDs flash dim/bright/dim. Incandescent lamps, unable to draw sufficient current through the resistors, remain extinguished until shunted by the transistor. Of course, those resistors can be cut off, but I did not do this as these are not my boards.
The Circuitron board is specifically designed for LEDs but it too has a positive supply and NPN transistors, creating the same no flash condition as Depotronics for the AF signal.
I was musing out loud that the LED bulb maker would need to specify either negative or positive bulb tip types, until my wife pointed out you only need negative tip, as bulb will work on AC as well (wife's a smart cookie, guess all my 45+ years of electronics must have rubbed off! 
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