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Calculators: Handheld: Unitrex 1640WD

Size (approx):

67mm x 130mm x 31mm (max) (w x h x d)
Weight 82g not including batteries.

Power: 9.0v DC using 1 x PP3 size battery.  It accepts an adapter (9V DC 500mW) through a socket on the left side just above the display.  The on/off switch is on the right side at the top.
Case: The case is two-piece black smooth plastic with a raised upper section (which helps accommodate the battery.  A printed black label sits in its own recess on the top panel.  The red display filter is angled and convex to act as an additional magnifying lens.  It gives a clean bright image with very limited angle of view.  The subtle coloured keys are short travel, positive click type that work well, although they rattle like a box of matches when shaken.   They are lozenge shaped with little indents to.  White text is printed around the keys for labels.
Display: 8 digit red LED with bubble lens and a ninth digit for negative and error indicators.
Features: Standard four functions with percentages, change sign, register exchange and four-function memory.
Age: 1978 (receipt for from Currys, Chippenham, UK on1 August 1978 showing a cost of £4.95.)
Manufacturer: Unitrex by Radofin, made in Hong Kong.  Serial number 1322 (very short!)
Comments: Cheap and cheerful calculator which feels very light but is nevertheless quite attractive.  The logic is sound except for the fixed decimal point bug and lack of square root.  This case design was used for a variety of brands.

Components:
Boards:
Construction: Cannot find a way to open this without inflicting damage - can anyone help?

Logic comments: The (C/ce) key is used to once cancel an entry whilst a second press clears the whole calculator
Input overflow is suppressed inputting an ninth digit is ignored
Negative numbers are shown with a "-” in the immediate left digit travelling into the far left (ninth) digit, thereby allowing full eight digit negative numbers.
There is an automatic constant on all four functions
The change sign function can be used in mid number entry.
Divide by zero shows “C” in the far left (ninth) digit and is recoverable using (C/ce).
Overflow shows the result and “C” (“E” if negative) in the far left (ninth) digit and is recoverable using (C/CE)
Ex is used to exchange the display contents with the last register so that (9)(/)(3)(EX)(=) gives the inverse “0.3333333”
Memory storage is not indicated – you have to remember it
The percentage function just divides the number by 100
This has a “zero decimals” bug:  try (0)(.)(0)(0)(+)(1)(=) gives “1.00” as an answer.  This pseudo fixed decimal point system remains until you multiply or divide.  The number of points can increase though.

box

The box measures 80mm x 163mm and is 35mm deep.  It is printed orange and black and one end has a (Currys?) sticker on it “C22 05 4187 6).  There is a robust card former inside to keep the whole box rigid and cushion the calculator so I assume it never had a cover.

manual

The manual measures 77mm x 128mm and is printed in one colour black throughout. It is a ten-page foldout format booklet.  Only basic instructions and examples are given.  Reference IB-0795-1

Note: nowhere in the manual is the make or model number mentioned so I presume this is a generic manual.