The serial numbers on these were originally inked on to the plate, and I'd look at the struts as mentioned.
If the plate has been sprayed over, there may still be a shadow of the number... Very slight as the paint would raised by the thickness of the ink.
Unless of course, someone did a good job and sanded the plate before spraying. But if they did a good job, the serial number would have been stamped back on.
Check the lyre. it might be on top of it.
Check the backside of the key slip.
Happy hunting!
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