Score awning operator
Unfortunately, I don't believe it will be possible to remove the cover from your older operator. Truth actually makes a new Encore style of your part, the 39-239B, which does have a removable cover (39-451, sold separately). I don't know for a fact but I would imagine that this is one of the reasons they decided to change its design.
We usually use our 33-079. This can be used with pretty much any Truth brand operator.
I also thought the #10 screw would fit. That's what we usually use for this operator. We also have the 33-016. I'm wondering if the hole in the sill is larger. When I find myself in that situation, I have to use toothpicks to make it tighter. What was the diameter of the old screws you took out?