In recent years, the 49ers have identified promising young backups and signed them to modest contract extensions despite their inexperience.

The strategy is a sound one. If the 49ers are correct in their assessment of the backup’s ability, they can lock up productive future starters on the cheap.

A potential problem? Those backups who do become productive starters could grow dissatisfied with their relatively skimpy contracts.

Such a case has developed with right guard Alex Boone, who has not been a part of the team’s voluntary offseason program and missed the first day of OTAs today, ESPN reported. Boone, 26, is unhappy with a contract that will pay him bases salaries of $2 million this season and $1.2 million in 2015.

In November 2011, Boone, a backup who had logged fewer than 200 career snaps, signed a four-year extension that had a maximum value of $9 million, with bonuses and escalators.

Boone has since outperformed his contract. He’s started all 38 games (including playoffs) since 2012 and been a Pro Bowl alternate each season.

However, Boone is not the only member of the offensive line who has outplayed his deal. Left tackle Joe Staley, who signed a six-year extension in 2009, will earn a 2.7 million base salary this season after earning Pro Bowl honors in each of the last three seasons. In addition, left guard Mike Iupati, a two-time Pro Bowler, will earn a $2.97 million base salary in the final year of his rookie deal.

It’s not known if Boone will skip a minicamp from June 17-19, the only mandatory event of the offseason.