Mikoyan-Gurevich MiG-19, various Soviet experimental variants

The Mikoyan-Gurevich MiG-19 was the first Soviet (and world's) supersonic fighter. As expected, many experimental versions were created, among them the following ones:

• Mikoyan-Gurevich SM-6: Two MiG-19S (according to some other sources they were initially MiG-19Ps, however in the blueprints we consulted, it was clearly a regular one, sans suffixe) converted into testbeds for the Grushin K-6 air-to-air missile which was originally intended for the Sukhoi T-3 interceptor which never went beyond the prototype stage. 
• Izdeliye SM-21: In 1956 the SM-2/V prototype was converted for the third time and was successfully tested with the APU-5 launch rails moved outboard to the drop tank hardpoints. Given that the various configurations of the SM-2 were given in Cyrillic alphabetical order, this version should've been called "SM-2/G", but in fact, the aircraft was known by the name given by MiG, Izdeliye SM-21 as a reference to the rockets it carried, the S-21. A funny possible explanation could be that the suffix letter G could be interpreted as "Gavno" (Russian word for "shit").
• Refuelling probe testbed: During the late 1950s various MiG-19s were fitted with aerial refuelling probes. Tests were carried out from Myasischev M-4, Ilyushin Il-28 and Tupolev Tu-16 bombers. At least 10 flights were made with various successful and unsuccessful attempts with one of them ending in an accident. They all were discarded as, by late 1956 it was clear that aerial refuelling was more important for long-range bombers than for fighters. Furthermore, Mikoyan-Gurevich was already working on a new generation of fighters that would outperform the MiG-19 by far, so the idea of aerial refuelling was abandoned.
• Mikoyan-Gurevich SM-20 & SM-20P: In order to verify Raduga Kh-20 missile's guidance system, two early MiG-19 were converted into avionics testbeds and designated Izdeliye SM-20. As those missiles were to be launched from a Tupolev Tu-95 bomber, those fighters were more like missile simulators than real aircraft. The first one was manned, hence the SM-20P designation (the letter "P" in this case stands for Russian "Peeloteerooyemyy" - flown by a pilot) while the second one was remotely controlled. By late 1957 both aircraft had made 27 test flights and the results were optimal so it was decided to remove the guidance system from the prototypes and attach them to the real Kh-20 missiles.
• Mikoyan-Gurevich SM-30: This version was a special Zero-length launch system equipped with PRD-22R short-duration burn booster rocket. It was intended to be able to take off from a tractor vehicle. However, as the system required an airfield for the fighter to land, and G forces were too strong, among other reasons,  the system was abandoned. 

Sources:

1. https://en.wikipedia.org/wiki/Mikoyan-Gurevich_MiG-19#Variants

2. Midland Publishing - Mikoyan-Gurevich MiG-19. The Soviet Union's First Production Supersonic Fighter

3. Salamander Books - The Complete Book of Fighters