This is just my opinion, but I would not use this roo for breeding.
I'm not a long-time Marans breeder; however I have many years experience breeding other breeds/species. A side sprig is a disqualification and therefore an undesireable trait. Side sprigs are the result of two dominant genes that must both be present in order to show up in the offspring. Carriers can go undetected for a long time until the "right" pairing is made. A bird with side sprigs will likely produce 50% or more side sprigs when crossed back to either parent.
It is very tempting to breed a bird with such a fault if otherwise they have excellent type and/or color, but using such an bird in your breeding program will only fix this trait in your flock. As gilavina says, you can use this roo to test mate in order to find other carriers, but that is a decision you will have to weigh against the time, energy, and expense to rear up chicks that you will possibly end up culling anyway. If you know or own the hen and cock who produced this brood, you can know that both are carriers of this trait.
I wouldn't repeat that mating for sure.
Is this a reply to my photos in the post just before yours? I'm not sure where the side sprig is that your talking about.