Isnt that a Stuh42? Early versions had muzzle brakes?
StuH 42, produced sometime between 11.43-03.44, just the muzzle brake has been taken from 10.5 cm leFH 18/40. Similarly, you could also use one from leFH 18/39 (or Wespe).
Here's a late war variant, with reinforced barrel recoil mechanism (from 9.44 →) that eliminated the need for muzzle brake, but due to its bigger size it could not be used with the cast mantlet and no coaxial could be mounted either;
The basic Strumgeschütz-Batterie strength (since late 1943) consisted three StuH 42 out of ten Strumgeschütz - or recommended minimum of fifth assault gun being a StuH.