John, yours is another one of those P Laminated Steel barreled Grade 2 guns.
Very unusual that it has a skeleton steel butt plate, but as others have said, others are known to exist.
Here's a Grade-0 No. 10165 that I've shown before, with some very unusual features including a SSBP. Note, there is no checkering at all... even on the butt within the SSBP.
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