Erwin, I have never seen documented evidence of a 351C in a 1969. I am of the belief that Any 1969 351 should be a 351W motor. Unless new docs have come to light.
A 351W 2V is not a very rare occurrence in a 1970 Mustang. there has never been documentation to support that early 'H' codes had the 351W and Later 'H' codes had the 351C. At least not to my knowledge. I have had many 1970 Mustangs and Cougars with 351W 2V and 351C 2V engines in them, and they have been from both early and late model year builds, from what I remember. I have documentation on All cars I have owned, I will try to make some time to double check if mine had an Early vs. Late difference..
There is contradiction in the information you have previously received, because if Ford ran out of 351W in very late 1969, then they would not have had to 'Use up their supply' of them in any 1970 car before instituting the 351C as a replacement, as they would not have had any supply Leftover for 1970 if they ran out in 69..
The 1970 351W-351C 2V topic is always controversial, because there is no supporting data on why they were both installed in 'H' code cars. the 351C was introduced at the Very Start of the 1970 Model year, at least in 4V form, and we currently have a 1969 Calendar Year Built 1970 Ranchero with 351C 2V, which the Torino-Ranchero platform also has the 2V Variations for 1970. It was an across the board Ford Car thing.
To support what I am saying, I currently have a 1970 Mustang 351C 2V Intake Manifold with a Date Code of [9H26] which translates to August 26th, 1969. Very Early in the 1970 model year Production.
One thing is Certain. ALL 1969 351 4V Cars were Windsors. ALL 1970 351 4V Cars were Clevelands.
Just My Opinion, some won't share it with me. But I have owned and seen a lot of these. Andrew