Snap-on Tools - This is a vintage Drill-Tap U.S.-Metric Converter Sliding Pull Card Calculator.
When you pull the inside card up or down each window will show the Drill Size needed for each Tap size - one window for US and the other window for Metric.
One side of the card is for 0" to 1/4" drills and the other side is for 1/4" to 1" drills.
It also has charts for Taper Pipe Threads, Straight Pipe Threads and a Wrench and Bolt Conversion Chart.
Part number is SS-537 and has the old style Snap-on Logo, it show a copyright date of 1974 and a printed date of 1-74.
It is new old stock but might have minor blemishes from long term storage.

It measures about 8 1/2" x 3".

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