Notches on SU butterflies

[ratwing]

Some time ago I bought what was supposed to be a pair of SU HIF6 carbs from a Rover V8 to rebuild and replace my old and probably worn ones but have only now noticed that they're not actually a pair, one has a plain round butterfly and a little brass 'jet' that's very slightly below the surface just like the ones on my engine at the moment but the other is different with a small notch in the bottom engine side that fits over a raised little brass 'jet'.
I haven't taken them apart now, there's no point if I can't use them and there may be other internal differences although they do look similar externally.
I can't see any mention of this difference in Hardcastle's SU tuning book and SU's website doesn't help either and to cap it all off the little identification tags are missing too.
I assume I've been sold 2 carbs from 2 different engines that won't actually work as a pair?
Edit - wrong author, it's Hammill not Hardcastle.

[DaveEFI]

Is the notch over the vacuum feed to the distributor?

[ratwing]

It's a nice idea, I thought that too until I realised the vac take off is a small drilling (to the left of the jet thing) which goes straight to a short piece of thin metal pipe pressed into the bottom of the casting - the hole is hidden by the butterfly when fully closed like in the pictures.

[ratwing]

I'd asked Burlen's about this difference and got this reply -

The brass part is designed for smoother idle tickover, as it introduces a small amount of fuel into the system. Once the vehicle is warm and the choke is disengaged, it no longer has any effect. The notches in the butterfly are for emissions control. These were more often used in USA spec cars, as the emissions control at the time was more stringent. It would be wise to ensure that the carbs are matched in this instance too.

So now I know. I've also since noticed the throttle levers and links aren't quite right either which makes me certain these were never a pair...

[ChrisJC]

Nice - thanks for posting. A useful nugget of info.

Chris.