Watson's Weston Meter Mystery

Help me solve this Weston Meter Mystery. I got this good ol' Weston Voltmeter at the Antique Telephone Collectors Ass'n show this weekend, and it works great on the 3V range. I want to do a little modifying. It has three binding posts on the top, 3V, 150V and 300VDC. I want to change the resistors on the 150 and/or 300VDC ranges to make them 15 and/or 30VDC. I took the four corner screws out but the case won't budge. I've attached a pic of it, with the enlarged screwhead that maybe isn't a screw head, or whatever, and posted it to ABSE. Maybe someone has had experience with getting these apart. Thanks for any info.

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My email address is whitelisted. *All* email sent to it goes directly to the trash unless you add NOSPAM in the Subject: line with other stuff. alondra101 hotmail.com Don't be ripped off by the big book dealers. Go to the URL that will give you a choice and save you money(up to half).

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You'll be glad you did! Just when you thought you had all this figured out, the gov't changed it:

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I have a Weston Microammeter, and the trick to it was to remove the binding post knobs on the face (meter surface), the range switch on the face, then the screws on the back (the zeroing knob remained as a part of the case), then remove the screws on the back. In your case, look for "standoff" bumpers on the back and see if there are screws hiding underneath. Seems to me that the back should come off; look for a parting line.

Also carefully and with minimal force, pry it apart when all screws (except for the "W" screws) are out, to see where the resistance point(s) may be - in other words, does it seem that the "W" points are holding? If it seems that the "W" points are the only remaining fasteners, try (a) soak 8 HR+ with lubricating screw-removal liquid (forget the name now and too lazy do dig it out), (b) make a mold of the head and from that mold, a screwdriver (plastic or low MP metal to form in mold). Align the home-made scerwdriver and tap lightly with with hammer (to help loosen scerw) while turning. This might work well enough to keep the head shape intact, even if of plastic. Worst case is to break the plastic "W" off and drill the head off; replace with a screw that has a head that matches the others.