The addition of a hole behind the rear plate is to give access for maintenance to the back side of the Idler Shaft and Speed Control Assemblies. The way pmateosian did his change has the plate needing to be removed by accessing the inside of the headstock to release it. Kind of defeats the purpose of having the access hole. There have been other posts about making an access hole patterned after the "B" Headstock one. That allows the second Name Plate JPG provided the link for to clip in place just like a "B" Headstock. The "B" Headstock shown below has the opening for the access with tabs (indicated by arrows) for locating the pins on the Name Plate Assembly. This hole cannot be made by simply using a hole saw as pmateosian did for his.
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Some have made that addition to their "A" Headstock by drilling holes around the desired diameter hole and filing the hole diameter smooth while leaving the tabs to locate the pins on the back of the Name Plate in the proper position. The hole needs to be the correct diameter for the clip to hold the Name Plate in place. I don't recall the correct diameter for the hole and would have to go out to the shops to measure one.
