Hello Rob!
Great progress here! This GAZ is really a classic!
I was puzzled by this rocket launcher, so I tried to look up the Russian net a little and here's what I found, I hope you'll find it interesting. Source is a book by V.N.Shunkov "Rocket weapons":
M-28 (MTV-280) and M-32 (MTV-320)LAUNCHERS FOR TURBOJET PROJECTILES
At the end of 1941, the command of the Leningrad Front, in preparation for breaking the blockade of Leningrad surrounded by German troops, instructed the engineers of the Leningrad artillery range (LAP) S.M. Serebryakov and M.N. Aleshkov to develop heavy rocket mines of high-explosive and incendiary action. The need for such mines arose due to the fact that, in the presence of a significant number of weapons for the destruction of enemy defensive structures, the Leningrad Front did not have enough ammunition for them. The task posed to the engineers was greatly facilitated by the fact that in mid-March Soviet troops operating in the Volkhov region seized a German ammunition depot in the village of Konduya, which also contained 28 Wurkorper Spr turbojet projectiles. (280 mm high-explosive mine) and 32 Wurkorper M.F1.50 (320 mm incendiary mine). Their design was taken as the basis for the creation of the Soviet turbojet projectiles M-28 (MTV-280) and M-32 (MTV-320). Used on the Leningrad front, the abbreviated name "MTB" (heavy rotating mine) reflects the principle of stabilizing the mine in flight: by rotating around the longitudinal axis due to powder gases flowing from the jet chamber through a nozzle turbine with 26 inclined holes of a complex profile. Both mines had approximately the same design and consisted of a body, a jet chamber with a turbine, an ignition nozzle, a head sleeve, an explosive charge (for the M-32, a flammable liquid), a detonator and a fuse. Both mines were equipped with the same 128 mm diameter turbojet engines. The diameter of the over-caliber part of the M-28 was 280 mm, and the M-32 - 320 mm. By July 1942, military representatives received 460 M-28 mines and 31 M-32 mines from Leningrad enterprises. The former were filled with an explosive "sinal", and the latter with a flammable liquid. Military tests were carried out on July 20, 1942 in combat conditions: 192 heavy M-28 mines (more than 12 tons of explosives and steel) covered two enemy battalions at once: Spanish volunteers from the Blue Division and the Germans who were replacing them at that hour in the fortified area of Staro-Panovo ... The shooting was carried out using launchers of the "frame" type, on which capping boxes with mines (four for each installation) were laid. The launcher consisted of a frame, opener, rack and mounting angles. With the help of the rack, the launcher frame could move in a vertical direction, which made it possible to give it different elevation angles. Traversing the installation was carried out by turning the frame. The capping boxes placed on the frame were wooden frames with the same outer dimensions. The inner dimensions of the boxes corresponded to the caliber of min. These boxes were used both for storing and transporting mines, and for launching them. The same principle was used to create the Soviet M-30 and M-31 rockets. In addition to frame-type launchers, by the beginning of the fall of 1942, a self-propelled six-shot launcher LAP-7 (LAP - Leningrad Artillery Range) was developed in Leningrad. Its artillery unit was mounted on a three-axle GAZ-AA truck. A characteristic feature of the LAP-7 was the transverse arrangement of the capping boxes, similar to that used in the MU-1 pilot plant. LAP-7 made it possible to separately elevate pairs of rocket mines, that is, it was possible to launch three pairs of shells at different elevation angles - to fire one salvo of three different targets. The M-28 and M-32 rocket mines and the installation for their launch did not become widespread, since in June 1942 the Red Army adopted a powerful M-30 mine with almost twice as large (2.8 km instead of 1.5 km) flight range. Nevertheless, 10 thousand mines M-28 and M-32, manufactured in besieged Leningrad, provided significant assistance to the Soviet troops in breaking the blockade of the city.
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Now this would explain why this is a relatively obscure piece of equipment - a "one off" by the standards of the Red Army.
Good luck with your build and have a nice day
Paweł