Well, from your original linked page, I see that C1 = AE235B (UNE) = St 37-2 K (DIN) = 10037 (W. Nr.º)
Going with the UNE number, AE235B, I find that it is equal to 1015 in the USA. Per:
http://www.splav.kharkov.com/steelgrade/mat_start_eu.php?name_id=4#1
I wonder why they said USA and not AISI. The 1015 is an AISI designation. Also I find it odd that on the original page linked, the AISI field in the chart was left blank. I guess a line by line comparison might turn up some discrepancy, but the cross-referencing I did seemed solid.