This mold made insulators with a MLOD and a mold line around their middle beneath the wire groove.

It was an auto-ejecting machine. It was said someimes the convergence molds would open and the insulator stick inside (hot metal clung to the mold). So with the stacked clamshell mold, they opened the bottom halves before the top halves, thus only the top of the insualtor could stick (if it was going to) and there was less chance of non-ejecting.

Sorry the pic is tilted - it is in my original PDF also! Note this is one instance where the insulator shown is not an accurate depiction of a known CD.