Now that's a great find! This is one of the best accessories.
I understand that Lodge also made a CI reversible griddle that fit the grid.
I don't have the nice Weber one, but I use a reversible CI griddle that I have right on top of the grill grate. I especially like it for shrimp, when I don't feel like skewering them. By the time you get the last one on, it's time to flip the first ones. Might just have to use it for some breakfast this weekend...
Congratulations again on the find, can't wait to see some cooks on it!