The engine code letters in a vin / serial number had a tendency to change from year to year as the displacement increased and newer engines & options became available.
Example: in '55 Ford cars the 292 was code letter P. In '56 the 312 was code P, and the 292 was (reduced to) code letter M. In '57 cars the 292 (2bbl) was code letter C.
ECO is not an engine block casting number for a Y-block.
It's likely a slightly lumpy marking of
ECG, a '55 thru '57 272.
The location of that number at the base of the distributor means the block was cast at the Dearborn Industrial Foundry, DIF.
Often found in trucks, which used a rear-sump oil pan, and were painted yellow as yours is.
(photo below)
http://www.ford-y-block.com/Block%20identification.htm
http://www.yblockguy.com/identify_y-block.htm
http://www.y-block.info/casting/blocks.html
http://www.y-block.info/enginecolor.html
.