Some information on the 12ft x 4ft Arthur Spurling Rowing Boat
The Spurling Rowboat is very famous and highly regarded. Plans may be available via mystic seaport, though I could not find them on their website. An image shows a set of plans by Thad Danielson, #1985.79, whether they still exist or not, I do not know. The second last boat has been stretched to 14ft.
The final boat is by Jarvis Newman who took the lines from the Spurling and made/makes a boat in fiberglass. Originally called the Jarvis 12, and now the Newman 12. Apparently many have been made.