I don't know for sure, I also did a lot of research on the GP100's used by PD's , foreign military forces and security companies. I have two 4" blue .38 GP100's, a half lug and a full lug and both were used security trade ins, from where I don't know. I had talked to a guy years ago on another forum who said he was a guard at a nuclear plant in Utah or something and they used .38 GP100's, I think he said this was back in the 90's.
I also have a 2003 4" .357 stainless, full lug adjustable sight GP100 that came from a Dept. of Energy trade in.
As for the Davidsons GP's, it's anyones guess who the contract was for. I know some states in the US only allow private security companies to issue .38 Special revolvers, but this may have changed. Rugers are also pretty popular in a lot of European countries, I know some French PD's had used .357 SP101's, but a law made them switch to .38's. Maybe the same type of thing, a Euro country that wanted .38's for their police ordered the GP100's.
I also know a lot of European counter-terrorist forces use GP100's, the French GIGN have loved .357 revolvers for close range use for decades and have been known to use the Manurhin .357 and the GP100. The Serbian and Czech Republic counter terror units also list the .357 GP100 as a weapon they use. I can see why, an unbreakable, durable revolver that has good close range stopping power, and pretty much no need for maintenance.