Bowers Boremike zero setting

I have several Bowers Boremikes, on one of them the zero is off by 0.0003". I bought these used and have no manuals. They are the
mechanical type with the plastic window over the micrometer barrel, this one covers the 3/4" to 1" range. When I check it against the ring gage it's reading high by 0.0003" so I'm guessing there must be some way of rotating the barrel or the spindle to compensate, but It does not seem obvious what to adjust. Anyone play with one of these?
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    Not with those, but look near the head for a hole for a micrometer pin spanner type of thing for rotating the OD of the shank. This is what is on my B&S/Tesa mechanical digital micrometer.
    In the B&S/Tesa tri-mics there is a setscrew which allows the thimble to be rotated IIRC.
    Good Luck,         DoN.
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I found two 0.050" hex set screws that when backed off allow the
shroud and plastic window over the barrel to be removed. Under the
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