I've seen a number of them - usually in the middle and upper grades. It was probably a special order item - like with Parkers. I don't know if I would call it rare, more unusual. I've seen fewer Smith with half-pistols on them.
Last year I just missed a nice, 16g 2E with a straight grip.
You can read more about what was offered by grade here:
http://www.lcsmith.org/shotguns/grades.htmlBTW: of all the major American makers, it seems like Lefever made a lot of guns with straight grips. But proportionally, I don't know if they more than the others.
OWD