Mick....that must be a small block...I've never seen that pin on any of my .21's or .28's....with that said did you remove the sleeve or anything? was that sticking up when you removed the head?.....

my best assumption would be that that is an allignment pin to allign the sleave into the block ( I am bettin that there is a notch in the sleeve were that is, and will also be one in the block also)....but if you didnt remove the sleeve it SHOULD be alligned....but if you did remove the sleeve you will need to make sure the allignment is correct before putting it back together..