The GHD marked stocks start showing up around the 935,xxx serial number range.   But there still are many WB marked stocks for a time thereafter until around the low 940,xxx serial number range.  After that almost all have GHD marked stocks with once in a great while a WB marked one that show up.  I do have data on several GHD marked guns that have heat shields with 6 rows of cooling holes in the 935,000 to 939,999 serial number range.  By about the 942,000 serial number range almost every 97 trench gun has the later type heat shield with 4 rows of cooling holes.  Hope this helps.