Typically with the 2' (solid) dish you have the feedhorn which has a post on it. You would align that post with the dish mount in the orientation that you prefer. Yes, to the side or facing horizontally, or up and down for vertical.
Otherwise you would have a dual polarity feed horn. Which you would go off of the location of the N connectors for your polarity.
There were also wideband single polarity feed horns. In that case as well you would mount the feedhorn and use the n connector as your reference point for polarity.
You mention screw holes, which leads me to believe you have one of the two later horns I mentioned. The first mentioned uses a jam nut on the back of the feedhorn/dish. You would loosen the nut and remove the feed horn to reposition polarity according to the previously mentioned post. But, the plate that the first feed horn uses is removed for the other two feedhorns mentioned.
Confused yet? LOL--