The tribal rivalry between Manly Marlins and Warringah Rats was sparked by a move that has been compared to the moment when Manly's favourite rugby league son, Bob Fulton, left the Sea Eagles to join their enemy -- Eastern Suburbs -- in 1977.
The Peninsula rivals face off again on Saturday, and, as ever, there is edge, animosity and get-squares revolving around the fixture; it will be an encounter to remind every administrator that tribalism, not teams cobbled together with silly names, drags in the punters.
This time around, the Rats, who haven't won at Manly Oval since 2007, will arrive at the cauldron for Sydney rugby's most intense duel with a special weapon.
The Rats' coaching entourage now includes one of the Marlins' heart-and-souls, with Damien Cummins having previously been involved with Manly since 1991; 'Turtle' moved through their colts system before playing 250 grade games, including their last first-grade premiership in 1997, followed by two seasons as first-grade coach.
Cummins' Marlins career involved numerous bitter encounters against Warringah, especially at Rat Park, where he was the target of abuse from the local cheer squad 'The Hillbillies' who treated him as if he were "a mass murderer". He headed north up the Peninsula this season after Manly opted for a change in coaching personnel, the Marlins giving the job to former Waratahs assistant coach Brian Melrose.
Cummins is certainly not the first Manly "diehard" to move to their arch-rivals.
Manly was the be-all and end-all of Peninsula Rugby for many decades, before several former players banded together to establish the Warringah club in the 1960s to provide an alternative for young players who lived between Dee Why and Palm Beach.
Warringah sat deeply in Manly's shadow for some time, until the famous coup in 1977 when they lured one of the Marlins' most recognised players -- Wallabies forward Tony Miller -- to take over as their coach. Miller had been ousted as Manly's first-grade coach, and the Rats seized the moment. In the 1980s, the Rats became a considerable force.
Manly's new coach is mindful that Cummins could be a destabilising influence on game day, but Melrose knows he won't have any problems psyching up his own players. They invariably lift when confronting the Rats, and they were belted by Warringah 43-17 at Rat Park last year.
Melrose played his club football with Parramatta, West Harbour and Eastwood, but he is well attuned to the intense Peninsula duel. He compares the rivalry to the time when Parramatta and Randwick were angry opposites during the 1980s.
"I've been at a few clubs over the years, and this match is probably bigger than any other round game, because of its history," Melrose tells ESPN.
Unbeaten Manly lead the competition, with Warringah in fourth spot, and Melrose has been enthused by the rise in club playing standards.
Melrose said the quality of play in the Sydney first-grade ranks was higher in the 1980s and 90s, which was understandable "because it involved the Test and state representatives" and then "club rugby went through a bit of a patch due to the dominance of some teams such as Sydney University, which is not their fault".
"Club rugby didn't die off, but there was a period where crowds went down a bit," Melrose says. "But the last couple of years, it has really picked up and you cannot find a round where any team could relax. The draw now looks tough every week, and that spreads the interest.
"On the back of a disappointing Super Rugby season, tribalism has become so important again."
Melrose was hoping to have one of his Waratahs squad members -- Matt Lucas - available to face the Rats, but lost him mid-week as the scrum-half has been beckoned to New Zealand to sit on the NSW bench against the Chiefs on Saturday.
"But now the evenness of the competition creates exciting games. Our games against Norths, Randwick and Eastwood have been very, very good. The match against Norths was as intense as you could get."
As a further indicator that Sydney club rugby is alive, well and kicking, officials are anticipating a capacity 8000 crowd at Manly Oval. Some Super Rugby teams are struggling to get similar numbers this season.
LIVE Saturday 3pm on 7TWO in New South Wales only. Early Sunday morning replay in Queensland on 7TWO.