Harold Hall Grinding Rest Question

[SteveM]

I am working on the grinding rest Chapter 8 Book 35, Milling, and have a question. Oa page 84 item 20 it shows the side arms and one has an oblong slot 8x16 mm(?). It's probably obvious but I don't understand why? Any help will be appreciated.

[Arbalist]

Check out this build log:

http://www.modelenginemaker.com/index.php?topic=4045.msg74621#msg74621