All that second letter (C3AE-6015-N) means is the first used or original application. The 289 block was first used on a full size Ford. Newer block were used on Fairlanes (C4OE-) and Falcons (C4DE-). There were no Mustang designed engine castings until the 69 Boss 302.
Jim

 

So in other words if I have a C5AE in my mustang fastback then it could very well be original? So there were not any C5ZE engines? The first Z engine should have read something similar to C9ZE ?

 

Located by the casting part number is the date code. For a car built for the early 66 model year, August, the date code would look like this - 5F21. 5 = year of casting, F = month, May and 21 the date. The casting would be three to five months earlier than your cars build date as Ford cast the blocks and let them 'rest' (the tossed them out doors in the snow) for a couple of months.
Jim

 

If I post some pics could you guys tell me what I've got? My block does not have the casting number in the correct place, I know-on passenger side under the cylinder above oil pan- nothing there.