Well I'm slightly surprised that you have no A at the beginning of your serial, but then you people whose slots face rear, hmmm, I don't know, but they do say you're a rum bunch at times, oooo.
507002401 means that the frame was built in July 1995, which pretty much for sure means that it was built for the 1996 model year.
The Orange website says that if your slot faces rear, that's consistent with earlier rather than later in the glorious history of the C16R and that your post was originally a 30.0. Whereas if your slot faced front your post would originally have been a 27.2 shimmed to 29.6.
So, although you haven't admitted it yet, I suspect that you have a shim in your tube. Nothing to be ashamed of mind.
But it won't be an original shim (we know all about the concept of original shim, don't we? No, don't go there) and as it won't be original, we don't know what size it is. And the size of post you need is a consequence of that which we don't know. So that's cleared that up then, good. Next!