TomS,
My theory is the Shah fully intended to purchase the RGM-66 at a later date and they simply made due with what they were supplied.
Also, the Standard launchers fitted to the BH.7mk5s were actually different than those fitted to Iran's long retired WWII-era destroyers. The launchers used on those are actually TWIN launhcers, in that there are two RIM-66Bs in each "box", one stacked infront of the other. On the front of said box there is a hinged door that lifts the forward missile up (so its exhaust doesn't hit the missile at the back of the box directly I assume).