Thanks very much. I try not to tinker too much with settings at this level, although on the occasions over the decades when I've had to, carefully following good advice has always led to good outcomes. So, I found the driver, and here is a setting, but I think this one is really about the long edge binding, DuplexNoTumble etc.:
*OpenGroup: BasicGrp/Basic
*OpenUI *Duplex/Two-Sided: PickOne
*DefaultDuplex: DuplexNoTumble
*Duplex None/Off: "<< >>setpagedevice"
*Duplex DuplexNoTumble/Long-Edge binding: "<< >>setpagedevice"
*Duplex DuplexTumble/Short-Edge binding: "<< >>setpagedevice"
*CloseUI: *Duplex
*OpenUI *CNSelectBy/Select by: PickOne
*DefaultCNSelectBy: PaperSource
*CNSelectBy PaperSource/Paper Source: "<<>>setpagedevice"
*CNSelectBy PaperType/Paper Type: "<<>>setpagedevice"
*CloseUI: *CNSelectBy
Then there is this setting on a stand-alone line, and am wondering if it changes the grayed-out double-sided default button to allow single-side printing, but seems doubtful due to the use of the word duplex instead of the terminology (double-sided) in the printer window. What do you think? Otherwise, the words double, double-sided, single, single-sided don't exist in the driver scripts. I suppose I could change it, test to see what happens and then change it back if it doesn't yield the desired result.
*CNNotChangeDuplex: True