That's not a head I've ever seen. ECG-(anything) should be 1955 but -T is a new one. The date code should be on the side between the exhaust ports. That might help narrow it down.Lee Reese of Omaha is the true expert on 1955 casting numbers but I don't know if he is on this forum.
John Mummert..... no time to work on my own Y-Blocks