"Invite only," an always-animated Boone cracked. "You can't just hop into that group. That's a tub club."
And he's not joking.
Seeing them together in a line of individual ice tubs was hardly surprising considering the San Francisco 49ers' tight-knit O-line and its cast of characters at each position. The Niners return a group that started every game together during last season's run to the Super Bowl, and they're determined to take that success even further.
"It's great. Any time you can have guys returning that you've played next to and you know how they play, you know how they think, it really helps the game speed up," Boone said. "It's a big advantage for us, but at the same time we kind of have a target on our back right now trying to continue to be the best and moving in that direction."
San Francisco's offensive line made major strides last year, and now there is an even bigger task ahead: protecting Colin Kaepernick from Day 1 in the quarterback's first full season as the starter.
Boone insists the line and Kaepernick will draw far more attention now.
Especially with all the hype Kaepernick generated during and after his breakout season last year with his high-profile offseason of appearances, events and awards.
Left tackle Joe Staley, left guard Mike Iupati, center Jonathan Goodwin, Boone and right tackle Anthony Davis know their job is to keep Kaepernick safe at all times. It's hardly a small chore, even on their own practice field.
"They're a group that works well together, good size," 49ers defensive coordinator Vic Fangio said. "You can tell that they've played together for an extended period of time. We still do fine against them."
During Tuesday morning's session, Staley and All-Pro defensive tackle Justin Smith kept their arms locked well after the play in a good-natured, competitive moment so typical of the early stages in training camp when everybody wants to make an impression.
Offensive coordinator Greg Roman realizes he is fortunate he already knows what to expect from all of his linemen day in and day out over the course of an NFL season.
"Any time you can get that continuity it's a bonus," Roman said Tuesday. "Everything is in front of us. We've got to earn it, every day at practice get better, and we've got a great group to do it with. They're working hard and paying their dues."
Coach Jim Harbaugh stressed from the start of camp the need to protect Kaepernick, who took over the starting job from Alex Smith in November and ran with it all the way to New Orleans and a 34-31 Super Bowl loss to Baltimore.
"Well, there's nobody that wants to see their own quarterback get hit too often," Harbaugh said. "So, we don't really talk scheme, about what we're going to do and not do. But I think that's a universal statement that nobody wants to see his quarterback get hit too often."
With the focused line eager to carry some momentum from last year into the upcoming season, there will be plenty of incentive to keep a good thing going.
"I love it," Davis said. "It's a dream come true to play with a tight group as talented as this one. Growing up, you stand out as an offensive lineman. Here, I feel like we all as a unit stand out. We've got a long way to go, a lot of work to do. They knew who we were last year. We're getting better. We're not just looking at our little trophies."
NOTES: While Michael Crabtree worked through some rehab exercises on his surgically repaired right Achilles' tendon in the weight room, Kyle Williams wasn't practicing and fellow wideouts A.J. Jenkins and Kassim Osgood each came off the field with apparent right leg issues. Jenkins had his right hamstring rubbed down. ... Goodwin was absent Tuesday. ... LB NaVorro Bowman also sat out. "He's fine," defensive coordinator Vic Fangio said. "He's just got a little something, just being cautious with it."... The 49ers waived/injured LB Darius Fleming, who tore the anterior cruciate ligament in his left knee Sunday after missing his 2012 rookie season with the same injury. San Francisco filled the void by signing LB Travis Johnson to a three-year contract. ... Newcomers WR Marlon Moore and rookie TE Vance McDonald continue to impress with their clutch catches and attacking styles against the defense in team drills. ... Kaepernick had a pair of pretty deep throws during one sequence of 11-on-11 work: a 30-yard completion to Anquan Boldin and a 45-yard touchdown pass to Vernon Davis a few plays later on which the TE beat S C.J. Spillman and LB Patrick Willis.