Sterling USA SUB preamp

Sterling USA SUB preamp

Hello from sunny Spain,
I have an american Sterling SUB since a few years. I removed the preamp to paint the body and I notice the preamp its a bit different from the others I have had (USA sterlings and SR5s). Also, the pickup poles are alnico. I found some pictures of different preamps in the usual resources but nothing like this one.
Can someone enlight me about this one? I believe they used the same electronics as the standard USA ones. Cheers!
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