From Lee Valley of Canada an interesting tool for setting the correct angle when sharpening a plane blade. This simple looking sharpening jig was inspired by the Stanley No. 50 honing gauge (Patent No. 605333).
The guide or gauge is used for honing bench plane blades that are slotted to accept a chip breaker. The Lee Valley version should fit original and replacement blades with a minimum 7/16" (11mm) wide slot.
The bevel angle is determined by the blade’s projection. On this guide, the registration ruler sets the blade for either a 30° honing angle or a 25° grinding angle. The barrel-shaped brass roller prevents skewing the blade edge, yet allows the blade edges to be cambered.
Made in Canada from brass, aluminium and stainless steel.

Or if you prefer; the following is taken from the original instruction sheet (circa late 1940s):

ACCURATE
The setting gauge ensures the correct honing angle (30°) and also checks the grinding angle (25°).
SIMPLE
An amateur will obtain results equal to those of a skilled craftsman.
SPEEDY
With this honing guide a plane iron is sharpened in a matter of a few seconds.
RELIABLE
There is nothing to get out of order. A plane iron sharpened with this Stanley Honing Gauge will hold its edge longer and produce better work. The gauge is invaluable to the craftsman and amateur alike. It will sharpen accurately all makes of slotted plane irons for metal bench planes and with its aid the utmost advantage can be obtained from a plane.