I found a supposed '68 SS396 El Camino which has a 10 bolt diff. The states it is matching nos. and original. I owned a 350HP SS396 back in the late '70's and it had a 12 bolt. I thought all ss396's had 12 bolts. Can someone advise if a 10 bolt could be correct?

If it's a real SS in 1968, it should have come with a 12 bolt. With 40 years of wear and tear, anything could have happened.
Most importantly, is the vin correct for a SS ? If it were a chevelle, the vin would start with 138 which means it's a Super Sport. Not really sure about the el caminos' though.

If it were a chevelle, the vin would start with 138 which means it's a Super Sport.

For 1968 elky, it is the same, vin starts with 138