Close coupled seal - flat or ring - info please.

[jimox]

Hi all, I've had to remove my close coupled toilet cistern to replace an hard to get at fill valve ( I've only done this sort of job once before, about 13 years ago on a loo in my sons house )
The doughnut washer underneath was not bad at all, intact and spongy, and luckily I seem to have got away with re-using it, touch wood.
I would have put a new one in but I'm not sure of which one to get as the one I've re-used looks like a rectangular section one - 110mm outside dia, 68mm inside dia. about 15mm thick.
So l thought I'll get a spare ,but while I was looking on the Internet I came across a circular section doughnut ,that apparently fits into a groove on the pan, I remembered that my pan has this circular half round groove and my doughnut had the same shape raised up all around it, presumably from being pushed into the groove.But I've no idea as to its size.
So, is the round section better and are they a standard size? I don't really want to split the cistern and pan to find out its size.
Or should I just get a 110mm/68mm/15mm thick one?
Any advice is most welcome.

 

[Exedon]

A clear picture of said pan would be useful.

 

[jimox]

Hi Exadon, thanks for the reply.
In hindsight I wish I'd took some when I'd broken it down but at the time I didnt realise that the half round groove was there to accept a same shape O-ring style doughnut, I believe they are Ideal Standard/Armitage Shanks make.
My loo as no identification names on it and like I wrote its ok for now (might not be if it starts to overflow) so I'm letting sleeping bogs lie.