I wrote a long reply yesterday and my computer went off line. i'll try again. the casting number is on top of block right in front of the head..The small nos. are app. 3/16 inch high and the big are app 3/8 inch, big difference. both blocks are good but the big is scarcer and preferred because it sometimes has bigger boost port. any other questions, my email is boat 990@aol.com. The 45 bunch are a very friendly "family" but may not have seen your post yesterday.