Regarding Arrow: A Modern Oil Field Engine in the December 2004 GEM: Some years ago, someone wrote relatively the same article and I responded in kind.
I've noticed them all over the country, and like you, I finally stopped to check one out. It was a Bell, and looked like an F-M, model unknown. I wrote to Bell Mfg. Co., Bowie, Texas, and they sent me a brochure on their engines. I can see quite a few differences between the two engines. The date on their letter is 1981. I have noticed a lot of these engines have been replaced by small electric motors.
P.O. Box 15
Weston, ID 83286