Looks like a callus or better known as the pressure sore. A lot of older dogs tend to get them. Areas are usually the elbow area or the hock area. My dog has one only one of her elbows because that is the side she ALWAYS likes to lay down on. So essentially, it's the boney areas where there is a lot of constant repetitive friction, and the body does it to protect the area.
Personally, I wouldn't worry about it too much.
If you are worried, when you're at the vet next mention it or make an appointment for it to take off any concern!
Hi, did you ever figure out what this was? I was looking up carpal pads on rear legs as my 10 month old Labrador has exactly the same thing as your German Shepherd, my vet said it may be a wart or callous but to keep an eye on it.