MG T-ABC

What bonnet design incorporated vents on the top as well as the sides. Was there a standard or was it a modification please?

It was an aftermarket modification. All bonnets were smooth except for the usual side louvers.

Thanks Steve. What would be the purpose? Would it have been inconjuction with engine upgrades/performance?

Some people feel it helps to cool the engine bay better. Others claim it doesn't make a noticeable difference. I know it does let engine bay heat out because a friend said he could feel the extra heat on his face when using Brooklands screens. But whether or not that was helping to cool the engine, I don't know. I do wonder if it would negatively affect the crankcase drought tube since it seems to rely on air rushing past the pipe to create a vacuum effect.

On my 7 style cars it made a huge difference.
This is a standard Birkin bonnet, never has any cooling problems.
This is a bonnet I made myself and this car was always marginal on cooling.In defense this was a fairly modified engine as it was my race car. Best on a dyno 103 Kw out of a normally aspirated 1600 engine.