When I assign it as gamepad key in the U-hid nano, I can't get it to detect at all. There are choices for "down action" and "up action"- pic attached- I'm not sure what to choose in those fields.There's a description of the various action choices at https://www.u-hid.com/home/u_config_utils.php, but I'd recommend trying "Normal Primary" for down and "Clear" for up with a gamepad button and shifter toggling disabled in MAME.
To disable toggling, upgrade to MAME v0.151 (or newer) and manually edit the polepos.cfg file using Notepad.
http://forum.arcadecontrols.com/index.php/topic,135345.0.html
Tested and confirmed working in MAME v0.262 and MAMEUIFX 0.152. Just add the text in red to the port tag=":IN0L" (shifter) line.
Quote
. . .
<input>
<port tag=":IN0L" type="P1_BUTTON1" mask="2" defvalue="2" value="0" toggle="no" />
<port tag=":BRAKE" type="P1_PEDAL2" mask="255" defvalue="0">
<newseq type="standard">
. . .
If the shifter switch works backward, edit the .cfg file to map the control to "NOT".
https://forum.arcadecontrols.com/index.php/topic,165173.msg1741846.html#msg1741846
If the steps in this post don't work for you, check out this thread for some other options.
https://forum.arcadecontrols.com/index.php/topic,165173.0.html
Scott
EDIT: ROMnames for Pole Position changed in v0.150 so you'll need to update the ROM too. Easiest way is to find a known version ROMset first then download the matching version of MAME.