ive seen it many many many times....mostly on pontiac and olds carbs..most early take off that havent been restored..ive had 100s of carbs go thru my hands..and ive seen it..so..i dunno..its not on every carb but its on enough its not something someone made up..and ive only seen it on 69 and 70 carbs
multiple people..including a well known..reputable carb restorer talking about it with pics...follow this link..theres some theories
http://forums.maxperformanceinc.com/...ight=ink+stamp
http://forums.maxperformanceinc.com/...ight=ink+stamp
you can see it on this 70 carb
http://www.ebay.com/itm/1970-7040505...5Y5Cuu&vxp=mtr
http://www.ebay.com/itm/Rochester-Qu...p2047675.l2557
http://www.ebay.com/itm/69-CHEVELLE-...lXnh7C&vxp=mtr
theres more..but i need some sleep