Those holes are the upper "breather" holes for the lower frame. The other "end" of those holes should be down by the bottom front engine bolsters, on the bottom of the frame. They often get clogged up with rust flakes.
Like Momo said, "tap testing" is a good way to check for thin spots, or put some rubber in the jaws of slip-joint pliers and firmly (not HARD) squeeze the frame along the runs to see if it crushes. The worst spots on the 750 frame are at the bottom of those engine bolsters, if they were painted or crudded shut. The frames are easy to fix, I've done several myself.
On the 1970-71 K1, many of the lower holes were forgotten in production (even some K2 bikes were this way, before it was over). There was a "quiet" service memo sent out to have dealers watch to see if the holes were missing at the lower bolsters, and the instructions were to drill a 7/32" (3mm) hole behind each one, if not present. More than one of the ones I did had water come out during this 'mod'. I usually took an oil squirt can and squirted a bunch of oil into the holes you show, until it oozed out the [new] bottom ones, hoping to extend the frame life. Now, when I have frames out of bikes, I spray CRC into the holes until it RUNS out of the lower ones, then turn the frame over and do it the other way. That will stop any rust, almost forever, right where it is.
