8 3/4 489 case snubber bolt size conundrum

Hi everybody:

This is suppose to be a 3/8s bolt that holds the snubber on I'm told...but on this one it's not. The bolt starts but then stops after one 360' turn... it's too tight and the threads aren't damaged in the case. Did I get a hold of a weird one? Does anyone have any clue what size this may be?