Swallow tails have been around since at least the mid-50's...
Carl Ekstrom built twin finned long boards in the 60's with a split tail...
The first modern/shortboard use of the swallow tail is attributed to Steve Lis' wide tailed kneeboard design, which was based on Ekstrom's split tail longboards...
But there's a hidden history behind the growing use -- and then loss of favor -- of the swallow tail on the pulled-in semi-guns of the early to mid-70's.
The deep V's of the late 60's, with their straight tail rocker, left a nasty spine behind the fin. That spine caused tracking in smaller, junkier waves...
Shapers tried to address this issue with things like the tri-plane V, which minimized the sharp spine behind the fin...
...and Morey-Pope introduced V in the middle of the board with a flat tail area with their transition-era Camel line of boards. An early version of "reverse V."
As boards got narrower and flatter, they maintained V through the end of the tail to loosen them up...
To help minimize the sticky spine behind the fin, shapers started using swallow tails, which, in effect, reduced the length of the spine behind the fin...
Here's an example of a modern Deep V replica shape that utilizes a swallow tail. You can clearly see much of the rear spine was removed by the swallow tail (the area depicted in red).
As design progressed in the mid-70's, tail rocker began to reappear behind the fin area. Shapers left that rear area flat, which resulted in "reverse V" ahead of the fin. The boards still had V, but the rear spine was missing, and the problem was largely resolved. The widespread use of the swallow tail was replaced by squash and roundpin tails...which were easier to shape, easier to glass, exhibited less cavitation in turns, and were less prone to damage.
(Proviso: This is just one design historian's perspective...)
A good story...back in 1967, Reno and I chased Brewer over to Maui
to get boards shaped. Reno got his done but RB kept putting off mine.
One morning, up drove McTavish, Nat, George, the Witzigs, Russell Hughes
and Ted Spencer. Dick and Bob got into a lengthy discussion about
everything. On the car were a couple of the wide tail, deep vee bottom
boards that looked pretty strange to us...the only thing I liked were
the beautifully foiled Greenough fins.
After they left, RB got fired up
and shaped my board but it wasn't like anything I had in mind. I wanted a
9-6 Pipeliner longboard but he had other ideas after talking to
McTavish. It ended up being a 8-6 vee bottom mini gun, the first of its
kind, and really the first shortboard among our group. A few days later, a
big North swell came and Honolua wasn't great but it was clean in the
morning. We all went out, Brewer lost his board first, then Buddy Boy
and finally Reno..all their brand new boards busted in half. Nat paddled
out, spun out on his first wave, broke the nose off his stringerless
board. I rode a little one in. The only guy who surfed good that day was
Russell, he had a more conventional shape.
They all went back to Oahu
for the Duke meet where Bob got one wave that blew everyone's minds.
Solid 10' West bowl lining into the inside, he went straight down and
banked that wide tail over and went straight back up...no one had ever
done that before. He got heaved but he showed us something radically
different on that turn and nothing was ever the same after that.
winded..sorry. But they all went back to Maui and scored some epic
Honolua. Nat ripped and you could see in the pictures that John or Paul
took, his patched nose. But the hero of the day was George on his mat.
It was clean, peeling 6'-7' in front of the cave and the surf mat ruled.
Never forgot that.