Is there a manufacture date on the seat that says what month it was made? I would go by that. I looked this up and Graco had this on their website. You may want to check with the manufacturer.
I haven't found a month in it yet, but I'll scour it further and then do the +6 from that month if I find it. Otherwise, yeah, I think I'll have to call Safety First with the model #. Just need the darned thing to get me through the winter!