first, from the MRBZ CD:

"note
Engine 602.962
vacuum modified for cost reasons"

I thought this means they made them cheaper, less massive, or less complicated etc. but may be you are right about other failure cost, but why they did not mentioned this directly, and why only for engine 602.xx not 603.xx

Second, the new style has the hex bolts visible in the front, may be that is why it is cheaper to make!!?