The hole can be hard to see on the black plastic backplates. I came out to the field on day and saw a couple of guys trying to fly a Cox-powered Something-Or-Other. They were going thru an awful lot of gyrations while prepping it to fly, so I went over to see if they needed anything. No, they were fine, just fueling up. Turns out, they remembered the old "Two-Nipple" metal backplates and thought this plastic one was defective. So they were pulling the needle valve out each time in order to let the fuel in.....
The Divot