Hi Hugo,
I have gone thru my refs as best as I could and this is my OPINION based on what I've put together...
Can't really seem to find a PAK 39 L/43 anywhere..... only L/48 (mounted on JgPz IV and 38). Now either there has been a mis-spelling somewhere OR it is a mistaken reference to the KwK 40 L/43 as carried by the PzIV F and Stug F...?
Expanding on the latter I wonder if VOMAG used leftover KwK 40 L/43s for the pre-production series (which wasn't going into battle anyway) modified to enable mounting on the JgPzIV thus somehow unofficially renaming them to PAK 39 L/43...? It actually seems the most realistic scenario to me.
Now regarding the muzzle brakes, the KwK 40 L/43 & L/48 (for PzIV) as well as PAK 39 L/48 (for JgPzIV) DEFINITELY came equipped with muzzle brakes. During combat use, production JgPzIV L/48s seemed to quickly "lose" their muzzle brakes due to the huge amounts of dust they kicked up while firing.
Also, all photos of the 0-series (pre-production) I've found either in books, photo collections, or online, show muzzle brakes. (I'm sure someone - Frenchy perhaps????? - will quickly dig up one without

)
In conclusion I would have to believe that Dragon's interpretation of the vehicle is correct and that Achtung Panzer's and Wikipedia's incosistencies are due to carried over spelling errors or misinterpretations.