They were just the stock units. Not lighter or better cooling - just the standard item bolted on on the production line.
If there are holes (like on the 32-35) it was probably for saving material (therefore making the item cheaper / lighter) which is always an aim for any manufacturer.
The early 36 type is just a hybrid crossover between the hole type and the one piece later 36. It must have been expensive to make, and I bet Ford couldn't wait to get the one piece (must have been cheaper, surely) rolling down the line. having all those extra studs and nuts, extra machining, assembly time etc, would have made the two piece items relatively more expensive.
All the above is based on pure guesswork, and is in no way based on knowing any facts, other than knowing how manufacturers think, especially when dealing with the volumes Ford were dealing with.
Mart.