Brunell Printed Prec Catalog Question

In the above catalog, he frequently lists the following or sim., where the numbers refer to the year and then the denomination of the stamp, and the letters are meant to indicate which stamp it is more accurately:

1930/32 1 1/2ff, 3s, 3sinv, 4t, 4tinv

I get: 1930 1 1/2c Facing Forward (Harding)and 4c Taft

But what is 3s ?

I have scanned my Scott and it is not the 3c bicentennial Washington, since that appears elsewhere in the Brunell catalog. Going forward and backward in Scott from the Harding/Taft which are 684 and 685, the best thing I can find (and which is often found as a precancel) is: 1932 720 3c Washington. This makes sense with the date, but what is "S" here? Does this stamp have a nickname that I don't know?

ANSWER That 3c is known as the Stuart, of course, hence 3s. Leaving this for anyone else who may find the reminder useful.

Cheers.