21 stud timing cover differences?

[36tudordeluxe]

Need another timing gear cover for my '36 flathead, are all 21 stud timing covers interchangeable and look the same? The one I currently have has 4 mounting holes for the distributor but distributor only has three mounting bolt holes. There's nothing wrong with the timing cover I have now, I just want another one for a timing pointer that I will install.