Awesome! Sounds like this is becoming more and more of a doable project. So question: have you had any experience/trouble popping in a 5 string pup on a 4 string? Doing any actual routing? And while I've got you, are you modifying the necks or building new ones?It is my experience that most fender style pickups with 8 poles cover 5 strings and no "valley" for the A string. There are a few exceptions … Gibson mudbuckers do not work well with 5 strings, and EMG dual rail split p style pickups have a problem with the middle A string, but Seymour Duncan work great. Sometimes I put a blank cover to hide the poles.
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And once again, another gorgeous instrument you have there. I'd pay good money for a J bass with a narrow neck like that!