This dresses up the back of your Hammond build (right? *YOU* don't see it much, but your audience might in a live performance!)
It also acts as a drill template...and if you want a paper template, we have one posted on our support page, The headphone hole is drilled for a 3.5mm jack, but if you use that to center your drill, the gold circle marks the size for the 1/4" (which is what I like :-)
The jack nuts hold the panel on, but a bit of glue probably wouldn't hurt. Works for the Hammond 12123 enclosure and the Lyra DIY IO PCB.