Recently, I've been wanting to try 12grams on the MiniMag I just got, because I'm really into the one-ball style of play (I'm a pump guy, what can I say

)
Problem is, I want to have the 12g changer vertical on my gun, and I can't tell if the my gun has a Vert ASA with a gas-thru, or just a gas-thru.
I can't seem to get the foregrip off, but I found that a lot of the parts were stuck due to age (This thing is S/N 9447, and hasn't been well maintained), and thought that maybe it's just stuck.
Any ideas? I don't want to absolutely destroy this part trying to undo something that won't undo.