Quoted from tfduda:
This is an odd piece especially given that I couldn't recall whether or not my machine has one. Indeed, my BOP doesn't have it either and I can't remember having to need it (although the foto above suggests someone thought it was necessary to add a screw in this location). Given the responses here and my impressions after browsing fotos online, it seems that some machines have it and others don't. Wonder if it was something included on early machines during initial production run and then left off later ones (or vice versa)...?
If I had to guess, it was added to later production games as a "bug fix" to airballs getting stuck in that area. I guess we could probably make a better educated guess on it if we compared dates of games with it and without. The fact that I didn't find it detailed in the Red book tends to support this hypothesis that it was added mid production run.