Odd looking small block heads.

I was looking at a small block the other day with some funny looking heads. The engine builder said they were off of an industrial 318 (which I'm not familiar with). Spark plugs were an 18mm thread instead of the common 14mm. There was also extra casting material in the area where you would normally see an X, J, or U. Anyone familiar with these?
Thanks, Dave.

Had a set a few years ago, heard some motor homes and H-D's had them as well.