There were several variations of the No 20 sight made over the years, even variation in the same years .
This is the only version of the No 20 that I have ever seen on a 1920 and all have had the elevator with the fine adjustment screw, the gun pictured is an early one. This sight came in a few different heights for different applications, you will need the tallest of the versions I have seen, I think all the 99's & 22's all took shorter versions, the shortest are on 1922 and 23-A Sporters where the barrel is integral with the receiver.
I think the one on the left is for a 1920 -
The two on the right here-