I found some materials which I think explain what this is and why it was done. In summary it is a device used to determine the stiffness of the board ultrasonically.
This is apparently important for glulams because they are made of various grades of boards with those being able to better resist tension used for the outer layers, and secondary quality boards used for the interior. So they have to rate the boards to plan how to best use the material.
I suspect the + / - markings on the boards are simply a pass/fail notation based on whether the board has met or exceeded some predetermined stiffness criteria.
A very detailed and comprehensive source is Nondestructive Evaluation of Wood Second Edition, July 2015, Robert Jon Ross, US Forest Service, which
summarizes information on nondestructive testing and evaluation of
wood... nondestructive evaluation is the science of identifying the
physical and mechanical properties of a piece of material without
altering its end-use capabilities and then using this information to
make decisions regarding appropriate applications.
This seems to fit glulam manufacturing needs pretty well.
The specific test being performed by the device shown is to determine the material's "flexural stiffness" which influences how resistant the wood is to stretching or bending. This seems to basically come down to the following formula:
Modulus of Elasticity= C2 ✕ density
where the device is used to measure the speed of vibrations (sound) C through the board by timing how long a pulse takes to travel end to end, as depicted here:
The first reference I found to be readable but does get extremely detailed & technical. I found another good description in "Moisture content effect on ultrasonic velocity...":
Ultrasonic testing consists effectively of the propagation of low
amplitude waves through a material to measure either or both the time
of travel and any change of intensity for a given distance.
Applications include distance gauging, flaw detection and measuring
parameters which are related to the material structure.
The propagation of ultrasonic waves in wood depends mainly on the
mechanical properties of the cell wall. The cell wall density is
reasonably constant, but the modulus of elasticity varies owing to
variations in the cell wall structure; hence, one can expect a range
of values for the velocity propagation property.
The value of the velocity of sound in a material depends on the
appropriate elastic modulus, which is characteristic of the nature of
the vibrational stress applied to it and also on the density of the
The document Timber grading machine using ultrasonic and density measurements had a useful diagram of a real-world setup:
And from Ultrasonic Measurements In Brazilian Hardwood a photo of what could be the same kit shown in the question:
which is a "SYLVATEST ultrasound device".
There is also this photo from the second source mentioned above:
And apparently a similar device is still made under the same brand.