The hole thru the bar above the attachment (from side to side) is to put a quilting guide thru. There should also be a hole at the same level on the back of the bar for the set screw to hold the guide in place.
This is real timely for me! I spent the last couple days cleaning up an old White 77 Rotary. Mine has all the attachments but was missing the set screw for the quilting guide. I spent hours looking thru all my "parts" machines for a screw that would fit.
And yeah, there are several different sizes of rotary attachments. I also spent a long time going thru a LOT of them to find the ones that would fit the White. I've had this machine for several years...I bought it for the cabinet. I decided yesterday to get it cleaned up & try it out. When I bought it, I ASSUMED that all rotary attachments were interchangeable & dumped them in a box with attachments from 3 or 4 other machines! Now that I've found the ones for the White, guess I need to sort the rest of them out. When I was going thru everything, I found a buttonholer that fits it, too!!!