At least through around 2004. I used to get Herters stuff in Owatonna, MN at the Cabelas overflow store just South of their main store and they had all sorts of Herters variety. That location closed but it was pretty much a Herters warehouse and bargain cave, They were also in the catalogs til roughly that time. You may have been able to get them later than that, but I never cared to pay attention to when they were totally discontinued. The corks were for sale at that location along with unpainted dekes, mismatched boxes, and seconds through the early 2000's. Corks had the standard #72 heads, wood keels, and tails.
TonyH would probably remember a better date when Cabelas discontinued the Herters decoy line.