That legendary 1971 tour to New Zealand, still the only time the tourists have beaten the All Blacks in a series, saw John Taylor emerge as a formidable Test openside.
Slattery was on that tour but had to play second fiddle, a sore throat on the morning of the third Test when he was given his chance ruled him out and Taylor kept his place for the fourth and final Test.
The Welshman, who was one of those rare breeds who could kick with the skill of a back, both from hand and the kicking tee, started all four Tests of that series, his only one with the Lions.
Taylor followed his conscience ahead of the 1974 series and refused to tour South Africa due to his views on apartheid.
But in his absence Slattery grabbed his chance with both hands and cemented his name in rugby folklore as part of the Invincibles.
Slattery started on the openside in all four Tests that series, indeed the Irishman lost his first ever match as a Lion but was unbeaten in every single one of the other 24 games he played for them.
With a Welshman and Irishman already covered on the list of contenders it feels right that the next great No.7 for the Lions came from Scotland.
Finlay Calder, the skipper of the 1989 tour to Australia, grabbed his own slice of history on that tour Down Under.
Not since 1899 had the Lions lost the first Test but recovered to win the series but in 1989 they did just that, an achievement that owed much to their no-nonsense captain.
A discussion of great No.7s cannot be complete without a mention of Peter Winterbottom, whose two Lions tours were a decade apart in 1983 and 1993.
And while the English great was never part of a successful Lions series, he stood out in New Zealand where his performances in defeat won him over to the locals.
Two further Englishmen deserve a mention as the game entered professionalism, teammates in the back-row for the 2003 World Cup win, Richard Hill and Neil Back found themselves rivals for the No.7 shirt on the successful 1997 tour to South Africa.
Hill, who will almost certainly come into contention for the greatest blindside flanker as well, was a man of many talents who went on three tours, playing five Tests and winning three of them.
His injury marked the turning point of the 2001 series but in 1997 he started the first two Tests against the Springboks with the No.7 jersey on his back.
Back meanwhile came off the bench in the first two Tests before starting the lost third game, although the World Cup winner did start alongside Hill in 2001 as well.
And finally this past summer Sam Warburton skippered the Lions to a first tour win in 16 years from the openside.
Injured for the decisive third Test in Sydney, Warburton had already made his presence felt on the tour with Sir Clive Woodward describing his performance in Melbourne in the second Test as the best he had ever seen in a Lions jersey.
Still only 25, the Wales skipper could well be at the very top of this list by the time he hangs up his boots.