Yep, it read 0V on the backside pins. Only 1 Mayfly to test with, (another on its way). I’ll look into batteries. I just passed by the Batteries Plus store for the CR1220.
I believe it’s Nuon brand, but has a “textured” backside: https://www.batteriesplus.com/battery/lithium/1220?q=cr1220
I’ll swing by the BestBuy or look for the Energizer brand on Amazon. Thanks again! -h