Good explanation.
The later 750-4 engines, and just about all production engine cases of the period, were made with a die casting process. This is done using steel "dies", basically a metal clamshell assembly with a space in between that is the shape of the part to be cast. There is still some sand though, some holes or internal galleries are formed by putting a sand piece in the dies before closing them.
The sand is a damp mix of pure silica sand and clay, when packed firmly the piece is quite strong (for a lump of damp sand, at least).
Pieces made in low numbers are usually done with sand casting. The casting cost per piece is much higher than with die casting but the dies are very costly to make. Even now some racing parts are sand cast, but racing engine technology has advanced to where most parts are made in a complicated backflow or vacuum casting process originally developed by Cosworth. Instead of puring metal into a mould, it is filled from the bottom either by pumping it in or by sucking it in with vacuum. The vacuum system helps eliminate any bubbles: they grow very large in the vacuum and tend to rise out of the metal, then when the vacuum is released any remaining bubbles shrink to insignificance.
