Dear Denizens of the Porch,

I've come across a Rabone rule that has two scales on the top front; "Spanner"
and "Tapping".  The rule is stamped No. 27 but finding a Rabone catalog online
rule No. 27 scales are similar for 3 edges but not for the top front. The
tapping scale makes some sense in that each labeled increment is a bit short in
length of the indicated measure. So "3/4" is about 5/8", "2" is struck at about
1 3/4".  I guess this would be the diameter of a hole drilled to be tapped to
the indicated I.D., but again that's a guess.  I have no clue as to what is
going on with the Spanner scale. Marks in increments of 1/4 are spaced about
3/8" apart.  Between these marks are struck lines that are sometimes somewhat
centered in the interval, sometimes far from.  Maybe unrelated?  My internet
search has come up blank.  Does someone here know how these scales were applied?

Thank you,

Kevin in too warm because I'm not from around here Virginia.

photos here:

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-72157715463748903/
and here:

https://www.flickr.com/photos/188990335@N.../50219280826/in/album-72157
715463748903/">https://www.flickr.com/photos/188990335@N.../50219280826/in/album
-72157715463748903/