I don't know his contract; but pre-draft signings are better than vets minimum and almost always involve guaranteed money.
If they were looking strictly for a back-up, they would have waited until later.
Don't get me wrong; if they can find better they will.
But Center is no longer a priority. They can address Center in the 5th round and later because they don't have to scramble to find an immediate starter quality guy.
I like the strategy.
I've said all along that the first 3 picks are probably going to be skill players: TE, WR, CB, maybe maybe OLB if a really good one unexpectedly fell to them. They've done enough in FA that they no longer have to reach for anything; they can stick close to their draft board. So that adds to the unpredictability of the Rams draft. But, listening to GMs, it is usually only in the 1st round when a guy on your board is a head and shoulders above other available players. Rather, most draft boards consist of clusters of players that are graded evenly enough that you have a choice at any given draft slot.
My guess is that where there is a choice, the pick will be a skill player for at least the first 3 picks, maybe 4 (if you consider an Edge rusher a skill player).