It was designed in 1936. Was adopted in 1939. On June 22 1941 in RKKA there were 47 Br-5 mortars (45 Serial+2 pre-production model)
On May 01, 1945- 48 units (Including 1 280mm B-33 mortar. B-33 was an experimental mortar)

Br-5 Features:
Caliber-279.4mm
weight-18,400kg(battle) 19,700 kg(transportation)
Barrel length-17 calibers
Shell weight-200kg (high-explosive) 246 kg (armor-piercing)
Range-10,950m
Initial shell speed - 420m/s
Rate of fire- 0.5 shots/min
Maximal elevation angle-60 deg.
Arc of fire - 8 deg.
Transport-Mechanical, self-propelled
Speed-12-15km/h (highway, unpaved road), 8km/h-impassibility
Guncrew-15 men

8 Separate super-heavy battalions (Suprime command-in-chief reserve) were equipped by Br-5 mortars. Each battalion had 6 mortars

The drawing of this mortar I'll load later. It looked like 203mm howitzer B-4 M1931