Center of bottom hole to top hole showing where 3.75 would get me (to short). I have no idea if they were drilled by a previous owner or not, there is a set of holes in the center that currently has the faceplate secured.Those look like hand drilled holes and are you measuring center to center?
Pulled a 400 series one out of storage and did my best measurement. Center to center 3 13/16.
Thanks Fred, that's about 3.8
Despite what my amp measures, I concur, 3.75 should be the correct dimension.
According to Wikipedia it looks like the amp fits in a 3U rack spacing, which would leave the bolt spread at 3.75 inches center to center.
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You plan on casing these or milling??