Update:
So this morning I filled up the M1A1 with a different juice (I had cleaned it out last night after it was time for a carto replacement). So I put in my new carto with no issues, and I filled it up through the hole on the side with my trusty syringe. After a complete fill, I noticed it was dripping through the carto a bit on the connector end, so I just put it on top of the bottle and went to take a shower. After getting cleaned up, I went to take it off the bottle and.... it's still dripping... I said WTH... so I though about it... and I put the screw back in the hole... and wham... no more dripping out the bottom.
So it looks like if you put thin juice in it you need to put that screw back immediately or very slow juice explosions may occur.