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National Semiconductor NS600

• Type: RPN Electronic Calculator
• Size: 5 x 2.75 inches / 13 x 7 cm
• Serial number: 198932
• Date of Manufacture: ca 1971

This isn't the exact unit, but this is the very model of the first calculator I ever laid my grubby little hands on as a child, in late 1971 or early 1972. This calculator has a 6-digit display; I also had access to a 9-digit version, the NS900.

This is an RPN calculator with a two-register stack. There's no enter key; you start by clearing the unit, keying a value into the X register and hitting [+] or [-] according as to whether it was a positive or negative number. Actually, you were just adding or subtracting from zero.

In addition to the power switch, there is another slide switch labelled "DECIMAL". Turning this on engages a small LED decimal point two places from the right. Beware, however, that this is not mathematically significant. If you multiply 1.50 by 2.50, the calculator will read 375.00. This lamp is a convenience for folks adding up a column of numbers.

I can't get the exact date off the chip without destroying the case, unfortunately.