Wanted: To borrow 30.8 or 30.9 seatpost

I've got a Cannondale System Six, all online resources tell me it should have either a 27.2 or 31.6 seatpost, but it doesn't. The guy I bought it from tried to sell me a 30.9 seatpost with it and I've seen a couple of forum posts mentioning 30.8 and 30.9 but nothing else. Cannondale haven't responded to my asking.
I don't want to splash out on the wrong size, anyone got one I can give a go at fitting? Either or both sizes

Pls

I know of one other guy with a System Six, and he has a 30.9 post in his.

I have a Giant 30.9 pin....

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For sale?

Yep. I remembered I had it when I saw your post. I replaced it with a 30.9 Thompson.

£10 it's yours.

I'll have to leave it for now, I haven't got the money. And I don't know if the frame fits, it's bigger than my usual hence not wanting to spend anything on it yet.
I'll see after pay day though. Cheers