Size (approx):

80mm (max) x 134mm x 42mm (max)  (w,h,d)
Weight 268g including internal rechargeable batteries.

Power:

6V DC with internal sealed batteries (4 AA size).  Accepts an adaptor/charger (Model 275, stated 10.6V AC, however mine works OK with a DC voltage of anywhere between 3V and 9V centre positive) through top side socket on the far left.  The on/off-charge switch is adjacent to the socket in its own recess panel.  Charge time is quoted at 4 hours.  There is a statement on the back "Caution: Calculator contains high voltage".

Case:

Two-piece matt plastic case in very dark grey with a thin light grey strip in between. Quite a thick, dumpy calculator with prominent feet.  The keyboard is surrounded by a brushed aluminium panel , the upper section of which is printed in black and blue and textured silver (for the logo).  A neutral (glass!) display filter is heavily inset and tilted at about 30 degrees.  They Klixon keys are short travel, loud click but feel quite nice.  There is a large metallic panel on the back with calculation examples.

Display:

8 digit orange gas-discharge Panaplex display with a ninth digit and extra light (neon with orange filter) for charge indication.

Features:

Four function with percentages and two function memory.  Normal floating and financial input mode. Semi-RPN.

Age:

1973 (Originally cost $159.95).

Manufacturer:

Keystone, Paramus, New Jersey 07652, USA.  Serial number 0168989

Comments:

Superb early compact calculator with rare display.  These displays are large and bright so it really is one to collect.  Silly about the strange "AC" label on the power socket though. The case is also of high quality - see the description below.  The logic is sound enough even though the archaic RPN takes a while to get used to.  Inside it is crammed full of components - a feat of engineering from the USA.

Components:

1 x cpu; Rockwell 40 pin staggered DIL, 0.6" width (covered with a large sticker)
1 x 9 digit panaplex flat faced gas discharge display; Panaplex II Burroughs KR85 BR09250M USA
15 x transistors
25 x diodes
14 x capacitors
58 x resistors
1x transformer
1 x miniature neon

Boards:

The main board (95017 REL 2) is held to the front via six screws.    The keyboard assembly (45920-1 Texas Instruments)  is attached via 13 strong wide wires and held to the front by four posts.  A small board near the power socket has two resistors and a diode.

Construction:

Remove the four screws from the back and hinge the rear section of the case off from the right hand side (looking at the back).  The batteries are attached to the back of the case and separated from the main circuit board by a piece of card.  This has only just kept the calculator from damage on my example with old leaky batteries.  Strangely enough the card is labelled with positioning for four AA alkaline batteries.  Watch out, both the mounting posts and the glass display filter are not fixed so can fall about easily.

Logic comments:

(C) is used once to clear and entry and a second time to clear the calculator

Input overflow is suppressed, inputting a ninth digit is ignored

There is automatic constant on multiply and divide only

Negative numbers are indicated with a "-" sign in the far left (ninth) digit, thereby allowing full eight digit negative numbers

Divide by zero shows a "C" in the far left (ninth) digit with zero result and is recoverable using (C)

Overflow shows the results and a "C" in the far left (ninth) digit  and is recoverable using (C)

Switching the memory switch to "on" automatically accumulates every number with the (=) key

When there is something in memory the far left (ninth) digit decimal point is on

The logic is semi Reverse Polish Notation and takes some getting used to; to do 3-7 key in (3)+(7)(-) to display "-4"

It suffers the pseudo fixed decimal bug: key in (1)(+)(.)(0)(0)(0)(0)(+) to give "1.0000" which remains a fixed four digit decimal number until you need more or use multiply or divide

The cover measures 134mm by 162mm and is 45-55mm deep with a convex top section.  It is made of padded metal with the metal edging - rather like a electric razor box of the same era.  When you hinge the lid up it doubles as a tilting leg.  There is a section in the felt-effect lined former for a matching pen as well.  Unfortunately missing from my example.  It opens by pressing in the catch on the front side.

Originally it looks like it had "Keystone" printed in silver on the front but mine has worn off.

Note: I had to clip out the defunct and leaky batteries from mine, but the calculator still operates on an adapter OK and the charge light still lights when the switch is in the charging position!