I'm not personally familiar with Logan equipment but there are generically two types of turrets, tailstock turrets and bed turrets. I haven't checked vintage Logan catalogs to see whether or not they made both types, but basically a tailstock turret looks like a typical tailstock except that the ram is replaced by a tool-holding turret. The typical tailstock turret has the same back-set adjustment feature as a normal tailstock does. A bed turret does not look like a tailstock, and usually has no means of adjustment for backset. You said that you were looking at a tailstock turret, but the photos that dlane (Derrick) posted are of a bed turret.
On the Atlas or Craftsman tailstock turrets for 10" or 12", the upper primary part was the same and the base or shoe was either 1.000" or 2.000" tall. And it was fairly simple to make a riser that would also convert the 10" to a 12", But on their bed turrets, although many parts interchange, it would have been much more time consuming and ticklish to make a spacer to convert the 10" to a 12". And converting the 12" to a 10" would have taken a fair amount of accurate machining on the base, and the conversion would have been permanent.
So the answer to your question depends upon whether what you are looking at is actually a tailstock turret or not. Best thing to do is to post some photos.