[V8 Bob]

The picture appears to be from '50 to '52, (but probably late'51 to '52 based on VanPelt's color data) because of the narrow-belt staggered water pumps (starting in '50) on the bronze car engines, and the cast 1/2 bells and rear sump oil pans on the green truck engines, last used in '52. The center channel on the car engine oil pans was on all '49-'53 Ford/'52-53 Merc center sump pans, both smooth sided and ribbed.
It looks like the trucks were tested with their cast bells attached, and used a shorter adapter, while the car engines had a longer adapter bolted to the block with a lower inspection cover, similar to the production '52-'53 Ford/'51-'53 Merc covers.