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

Mikoyan-Gurevich MiG-19, part one, MiG-19 sans suffixe

In 1950 the Mikoyan-Gurevich design bureau, began working on a new fighter which would have a greater range than both the MiG-15 and the MiG-17, and would also have supersonic level flight capabilities.
Consequent work resulted in both the Mikoyan-Gurevich SM-2 (AKA I-360) and the I-370 which was developed in parallel.
The MiG SM-2, which was powered by two Mikulin A5MF jet engines, suffered from serious handling problems and, as the engine generated an inadequate amount of thrust, the Mikulin design bureau designed and developed a new engine to replace it, the AM-9B (later known as the Tumansky RD-9). This new engine was rated at 5.700 lbf (2.585 kg) of thrust power at dry, and 7.160 lbf (3.248 kg) with afterburner. A single SM-2/2 was refitted with this engine, received the denomination of SM-9/1 and flew for the first time on 5th January 1954. Its performance impressed Soviet authorities and it was therefore, ordered into production as the Mikoyan-Gurevich MiG-19 on 17th February 1954, despite factory testing had barely begin.
This first production model served exclusively with the VVS (Soviet Air Forces) and 401 machines were manufactured at Gorky's Aircraft Factory No.21 and Novosibirsk Aircraft Factory No.153 between 1955 and 1957.
As the aircraft was rushed into production, it had serious problems. The type suffered in-flight explosions which were eventually traced back to the poor insulation between the aircraft's engines and the fuel tanks in the rear fuselage which exploded as they were overheated by the engine. This problem was partially solved by installing a heat shield between the engines and the tanks. The aircraft's elevators proved ineffective at supersonic speeds and all-moving slab-tail was tested in the second and third SM-9 prototypes.
The new fighter was delivered to the VVS in June 1955, with the type being unveiled to the public on 3rd July, and 48 MiG-19s taking part in a flypast during an air show at Tushino airfield, in Moscow. The NATO assigned the codename of 'Farmer-A'.

Sources:
1. https://en.wikipedia.org/wiki/Mikoyan-Gurevich_MiG-19
2. http://www.aviastar.org/air/russia/mig-19.php
3. https://www.valka.cz/Mikojan-Gurevic-MiG-19-kod-NATO-Farmer-A-t81703
4. https://www.sas.upenn.edu/~jasonrw/mig19.htm
5. https://pickledwings.wordpress.com/mikoyan-gurevich-mig-19-farmer-the-forgotten-mig/
6. Midland Publishing - Mikoyan-Gurevich MiG-19 - The Soviet Union's First Production Supersonic Fighter
7. Salamander Books - The Complete Book of Fighters