A friend and I took a trip a couple hours west to look at a motor for his car. He has a project 69 Mustang (not a mach 1 but one that looks just like one). Anyway, among the things needed is a motor. My friend came across an advertised 69 351 windsor which we bought today. The question is casting numbers. It has the numbers C90E-6015 stamped on it indicating a 69 351 windsor. However there should be a letter after it indicating which car line it was to be installed in. This one has no final letter. Any ideas why this would be? Any other useful info would be great. It does check out to be a 351 by the intake manifold width and deck height.