At least it is not a test of the camera or the lens. The Leica IIIa is ... well, a Leica. The Jupiter-8 is a superlative 6-element Sonnar design which the Russians appropriated along with the whole Zeiss establishment as reparations after WWII.
The question to be addressed is that of compatibility. There is a lot of angst expressed in on line forums about slight differences in the lens mount to focal plane distances between Leicas and their Soviet counterparts. People report making precise measurements with their micrometers and even shimming their Soviet lenses to compensate for the perceived problem. So, when I confront a matchup of my Leica with a new Soviet lens, I make sure I shoot at a variety of distances to assure myself that both the lens and the camera are performing up to the expected standards. The following shots are from a roll of TMAX shot on a recent neighborhood walkabout.
I'm not seeing a problem. In fact, I've shot several Russian lenses with the Leica including FED, Jupiter and Industar models in 50mm and 35mm focal lengths without any loss of sharpness that is apparent to my eyes. There may be a real problem lurking out there and maybe I'll encounter it one day, but I'm not likely to lose any sleep over the possibility. I also have failed to find any apparent difference from the results I get when the Soviet lenses are mounted on the Soviet cameras they were built for.
I've had a Jupiter-8 lens for my Contax-copy Kiev IIa for a long time. That lens is about ten years older than my recently-acquired Leica-Thread-Mount Jupiter, but it is also an excellent performer. I haven't used it much, partly because I usually prefer to get out with the the Jupiter-12 35mm lens on the Kiev. However, The Kiev also does not offer the same level of compact precision as the Barnack Leica.
Besides adding a nice tactile dimension to the shooting experience, the Leica's slick operation instills confidence while also providing some practical enhancement to my shooting results. For instance, that buttery-smooth film advance mechanism yields a strip of exposures that are very narrowly separated and perfectly spaced, with the result that I can often get 25 frames from a 24-shot roll even with the long tapered leader the Leica requires.
The bottom line for me on the issue of the compatibility of the Leica and Soviet lenses is that the combination provides a practical and economical way to the Leica experience. If the Leica were my only camera, I would be more concerned with the likelihood that there could be a problem with lenses with a focal length greater than 50mm. Obviously, I have a lot of other choices of cameras if I feel the need to use long lenses.