I do not have any D1.6 books.
Is the prequalified base metal tables similar to AWS D1.1?
Do the specification columns say anything about type or grade?
ASTM 240 Type ?
Does it list UNS S41300?
I did find some tech. data from Columbus Stainless. It states that it is a 12% chromium utility ferritic stainless steel. I can only assume the D1.6 would use the more common 300 series, austenitic stainless steels. But you know how assumptions sometimes turn out. Here is that data I found if you are interested in looking at it.
http://www.columbus.co.za/download/downloadmain.htm
I only have the 1999 version, but according to Annex B, the only prequalified steels are those that nominally austenitic and normally produce a small amount of delta ferrite in autogenour welds. So no ferritic stainless steels are pre-qualified.
As for the pre-qualified table (3.2), you first look for the alloy designation. If you are looking for 304L (the first line in the table), it is prequalified if the 304L is certified to be A167, A213, A240, A249, or A276, A312, A403, A409, A473, A774, A778, A813, A814, or A851. If it is 304L to some other specification, it is not prequalified. The fact that A240, or A312 contain other alloy desingations is not relavent unless that alloy designation has the same ASTM spec listed in the same table.