I have found these strips. They have no part numbers on them, but the metal on the anode is marked with a black
+ on the top side centered next to the strip (not visible in the pictures).
This is how the strips look.
And in this picture you can find that each strip has 18 LEDs. To give an estimate of the brightness, that picture was taken with ISO 100, a Canon EOS 400D, the kit lens, aperture of 5.6 and an exposure time of 1/4000s.
I made a few tests with those LEDs to get a first good guess at their specifications. First, I started to figure out the voltage range at which the strip starts to light. To do this, I limited the current of my lab supply to 20 mA and gradually increased voltage until an effect was visible.
At 7V, the strip starts to light visibly, but not very bright. At 8V, it already draws the full 20 mA current my lab supply allows it to and is already considerably bright, already enough to make my eyes hurt slightly in a medium-lit interior room.
Looking at the voltage of white LEDs, I guessed that each LED might have about 3V of operating voltage. Judging from the above figures, I assume that the LEDs are arranged in a 3×6 pattern, that is, six sub-strips with three LEDs in series each.
To confirm that theory, I allowed the lab supply to supply up to 120 mA current to a strip and went up to 9V. A single (cold) strip draws 90 mA of current at that point. Letting the strip run for several minutes lets the draw increase up to 115 mA, presumably this would increase further if I gave it more time.
Hopefully, this information is helpful to answer the following questions about this part:
Ideally: What is the part number and the manufacturer, or even better, a link to the datasheet?
Less ideally, but also very appreciated: A good and reasonable guess on the actual specifications, that is: At which operating voltage and current do these strips normally run? I assume that someone more familiar with LED strips than I am might be able to guess some of this from conventions and my measurements.
If more data is needed, let me know.