The meter pictured I am 99% sure is an old HP 415 series, the last of the series being the 415E I believe. If you Google HP415E you will get hits like this one. Plus other hits explaining each setup in detail. I used that meter extensively back in the late 70's and early 80's which was the last I worked with microwave. Really a classic meter of the day.
On page 9 of the link you will see just about the same image you posted with explanations.