ah alright, because I saw sifting through the catalogues that when they weren't special order, EM bikes came in even + 53, 55 and 57 (?). I couldn't see the Alu Mega explicitly being only even sizes, so I assumed it was possible it was a 53 still.

To get more on topic, from reading the thread it looks like it's a late 97 (according to the vertical sn)/early 98(according to the EMC sn), is that correct ?

EDIT : the greater idea about the size was to be able to follow the catalogues' size chart to be able to figure out seat tube/head tube angles. according to them, a 54 measures 54,8cm on the top tube and 73,3° seat tube angle (which I assume to be the same for the head tube)