Even back in 1959 the plywood they used for the 16-footers was 4 mm., which is really light for what is essentially an open boat. As a comparison, most plywood International 14's of the 1960's (there weren't many - most were cold molded designs) used 6 mm. On Evelyn, the 16-footer, I count twelve stringers over the hull bottom (plus the center plank).
You see a similar surfeit of fore-aft stringers in "Vitamin C", one of the last plywood champion 12 foot Cherubs of the early 1970's.
Photo courtesy of Andrew Chapman
"Vitamin C" on a ripping tight spinnaker reach.
Photo courtesy of Andrew Chapman