which were the first Soviet types with variable pitch propellers, and when were constant speed units introduced?
First variable pitch propellers on series Soviet types were
Hamilton Standard propellers made under licence.
It looks that licences for several variable pitch Hamilton Standard designs were bought around 1937. All I-16 fighters, SB bombers, DB-3 bombers and PS-84 transports (licence made DC-3) made after 1937 had engines designed in US or France and variable pitch propellers designed by Hamilton Standard.
Constant speed governor (in Russian RPO for Regulator Postoyannih Oborotov) was also a Hamilton Standard design. Governors made under licence were known in USSR as
R-2. Redesigned governor
R-7 was used with VISh-61 propellers installed on early Yak-1. This is how propeller controls are described in Pe-2 manual:
Винтомоторная группа самолета снабжена металлическими трехлопастными винтами с изменяемым в полете шагом (АВ-5ЛВ-139, ВИШ-105П или ВИШ-61). Эти винты при помощи, регулятора постоянных оборотов Р-7 могут менять свой шаг автоматически и принудительно. Летчик может устанавливать различное число оборотов винтов путем затяжки пружины регулятора при помощи штурвала управления, благодаря чему угол установки лопастей винта уменьшается или увеличивается
Power plant is equipped with a metal three-blade in-flight variable pitch propeller (AV-5LV-139, VISh-105P or VISh-61). These propellers using the "constant revolutions regulator" (i.e. constant speed governor) R-7 can change their pitch automatically or by pilot's command. The pilot can set a different number of revolutions of the propeller by tightening the regulator spring with the control wheel, so that the installation angle of the propeller blade decreases or increases
HTH,
KL