No construction in the area traveled between checking pressure friday afternoon and getting flat saturday afternoon.
Sidewall gives max load at 35 psi and says "never inflate beyond 40 psi"
So the 31 psi I was running seemed reasonable.
After the first flat, tire was still in good shape and I found the torn stem before I took it to the shop. Then the shop said they couldn't find any other problems, and mounted it on the spare carrier.
2nd flat was a different wheel & tire. I just checked it and see no evidence of a puncture. I guess something could have gone thru the sidewall near the treadline...hard to tell since tire is destroyed from having to ride on the flat a while.
2 right rears, 2 different wheels/tires, seems too much of a coincidence for me. At first I thought I may have let the pressure get too low causing the first one. So I was keeping a close eye on pressure after that.
My engineering personality wants to understand why.