Actually, it's the number "2". It says "UL Series 2", and also, there is a visual difference due to the change in the grill material.The S2 has a 'Series II' printed right below the Epi badge on the front grill. The I just says Epifani.
I don't think there is any way to tell the difference between the original S2's and the heavier newer ones other than picking them up.
The newer models have the bigger grill material like the current Performance series (though you can't tell from their website, because it's so woefully out of date). Here's a pic with one of each.

