John, I saw the vin stamping info in a Pontiac service manual online. But looking at the pics in my '66 Buick manual, I don't see the pad. It could be stamped anywhere!
The '66 Buick manual does have a pic of the ID tag, It's HEG AD, with 10269 on the second line.
The throwout bearing assembly was used on MANY years of Buicks:
Group 0.799 pn 1367058
Used on 1939-48, 53-60, 63-66 Nailheads