OK, after readings Gary's last post, here's my issues with K.A.R.
During 1969, 1970, 1971 there was an M code engine option. It was a 351 V8 with 4 barrel Autolite/Motorcraft 4300A carburetor, 11:1 advertised compression, and ostensibly 300 BHP. Ford doesn't specify where that engine was manufactured.
The earliest 351C "production" castings more or less had August 1969 date codes (9H). Apparently there are "some" 4V heads out there with late July 1969 casting dates (9G22 specifically).
Here's a link to an old spread sheet for 4V cylinder head castings, including dates. The data was collected from the members of the Clevelands Forever Forum: 4V Cylinder Head Spread Sheet
Ford's production year is a fiscal year. This is where confusion often arises. The 1969 production year ended approximately June 30, 1969. As of approximately July 1, 1969 Ford geared-up to begin production of 1970 models. In spite of what Doug has written, a 1970 model year part, such as a D0AE block, can and frequently does have a casting date of the previous year ... like 9H (9 meaning 1969, H meaning August) in this example. Check-out the cylinder head data linked above for many examples. The explanation is simply that the 1970 model year began mid-1969.
However for a 1969 Mustang to have been factory equipped with a 351C was basically impossible ... since "production" 351C engine parts weren't being cast until August 1969. I know beyond a doubt that the "early" 1970 Mustangs ordered with M code engines were equipped with 351W 4V engines. Those 1970 models went on display at dealerships in September 1969, and were thus manufactured sometime in July 1969.
Unless of course those 1969 Mustangs were equipped with engines manufactured from SK & XE parts, which I strongly doubt. The experimental (SK & XE) 351C parts, which were also race parts, pre-dated the production parts by YEARS. But that's a whole different batch of parts from the production parts.
One last tidbit of history and then I'm off. The first production engine to be manufactured with Cleveland cylinder heads was not the 351C, it was the 1969 Boss 302. That car was late going into production, the 1969 Boss 302 Mustangs were manufactured in approximately Spring 1969. The Boss 302 engines pre-date the production of the first 351C by about 6 months. But the Boss 302 engine castings (including the heads) were not cast at the Cleveland casting plant, nor were the Boss 302 engines assembled at Cleveland Engine Plant #2.
The 351C 4V ad below, from a Fall 1969 magazine, says you can order the "new" 351 4V for your 1970 Torino, Montego, Mustang or Cougar. It makes it seem Donnie Allison won the Alanta 500 with a car powered by a 351C, but it really only says they were testing "better ideas" found in the 351C with their race cars.