Marty they are 10 or 20mm. I am impressed that your 3/8 stone is now 1/2 in thick. That means it has grown by about 3.2mm or 1/8 since you got it :)
As has been said, unless you are going to do a lot of work with it the thinner stones will last a long time, though the thicker ones do give you a bit more clearance in use.