I occasionally hear of these Just what is the difference. Just rebuilt my 455. Thank you
AL

 

There are notches on one end of each cover. They provide clearance for the HVAC box and the power booster in some models with the BB.

Oh...and people install them backwards all the time, You'll know what I mean when you have seen a few photos of it.

Here is a link for some new ones, just to get an idea what they look like. These are not factory original but pretty close.

http://www.ebay.com/itm/Notched-valv...sories&vxp=mtr