The left side, center, of the block should have a number cast into the side. Mine is 1HA6050. The top of the head should also have another similar number indicating the type of sleeves. There was an "H" series and a "G" series. I think the "G" series started with 0HA----. They next went to an OHV version.

Try this link: http://www.vanpeltsales.com/FH_web/flathead_six-cyl.htm