Question about the M7B1(Priest)

[zufzuf]

I am not too familiar with WWII equipment and I have a Priest in my current battle. I have two questions and maybe the experts can answer them.

What is that second tube above the gun? I tried searching on the net but found no info. And how should one use this asset in the game? Is it only against infantry and light vehicles?

Thanks in advance,
zufzuf