Hello! Can anyone help me?

I've decided, after nine months of commuting 10 miles a day on my bike and loving every second, to invest in an exciting new saddle. I've settled on a nice springy Brooks, but I don't know whether I need a Brooks B66 or a B67. Apparently it's all about the type of seat post you have, but google hasn't so far presented me with an explanation simple enough for my un-technical mind to comprehend.

I ride a Raleigh Cameo - it was made in Nottingham, and I think it must have been made not long before they started exporting their manufacture. Sorry I can't be more specific, but when I bought it I was told it was about 20 years old, so presumably it was made around the late '80s/early '90s.

If anyone happens to know what kind of seat post my bike has, or failing that, what exactly I should be looking for, I'd be very grateful!

Thanks!

Grace