This is a area I've explored before although it stems from an article which was first published in 1975. Following the link below you'll see it was updated a few years ago by James Oberg.
Although conjecture on my part Leonard Stringfield's source stated that his primary task was the translation of intercepted Soviet military transmissions, during which he recalled one particular instance when decoding a message received at the ships ‘crypto-machine room'.The intercepted transmission related to a Unidentified object over-flying Europe.
Initially I took this to mean that the Soviets were tracking a "Unidentified" Object over Europe, now "What If" the object was actually known to the Soviets but the objects true origin was lost in translation leaving the Spyship to classify the object as Unidentified?
Whilst today much of the former USSR's Space program is now in the public domain we can see from the article that in the years leading up to Yuri Gagarin's successful manned space flight in 1961 the question of mystery surrounding possible missing or failed (manned) space flights appear to have been answered although could the same be said of unsuccessful unmanned space flights during the early 1960's?
In some respects the notion of a space vehicle going astray would lend credence to Stringfield's source and South's testimony regarding a partially covered Delta wing craft being loaded onto a flatbed truck. Its not beyond the realms of possibility that the vehicle in question could have been an early prototype unmanned Russian version akin to NASA's lifting body program.
Equally though the drawbacks to this speculative theory are themselves in abundance, officially the History of USSR's Space Program for the period in question tells us that with the success of Sputnik 1 in 1957, the USSR had a considerable number of ambitious projects under development with the goal of sending space probes to the Moon Mars and Venus. Certainly by the end of the 1950’s other Design engineers had entered the program with an additional further array of ambitions - including Manned Spaceflight to both the Moon and Mars, Orbiting Battle Stations and Combat Spacecraft.
By the beginning of 1960 there were no fewer than thirty Space Systems on the drawing board, admittedly few of these designs ever reached the hardware stage largely because of internal wrangling that existed within both the political and military mindset of the Soviet Union. Nevertheless the Military overall asserted control, with the research programs of 1962 to 1964 code-named Shchit (space systems), Osnova (space equipment), and Ediniy KIK (ground systems) thus defining what would become the first generation of Soviet operational space systems. Although many of these systems would not be realised until 1966.
Arguably one of the most contentious projects during this period was the considered need for a Space Plane and while work continued on winged spacecraft for launch by ballistic missiles. The first such program, the VKA of 1958-1960, resulted in developing designs launched by means of the Korolev R-7 rockets. In 1960. this work was cancelled, and development commenced on Kosmoplan and Raketoplan space planes. Whilst ultimately these projects would also be cancelled there nevertheless - was a considerable amount of research and development afforded the concept - while officially no manned version of the VKA was ever manufactured.
Would it be considered to outrageous to ponder the question… was there an ill-fated unmanned prototype version of this winged spacecraft carried aloft on a Korolev R-7 Rocket that possibly malfunctioned whilst in orbit back in 1964?