Drain plug washer dimensions

Hi Folks,
I'm just about to do my first oil change on my 1980 1600 gli. Could someone please tell me the internal and external diameters of the drain plug washer ? I'd like to avoid dashing off to the spares shop mid oil-change.

Many thanks.

Do they not put them in with a new oil filter these days???

Ah - I'll have a look. I'm at work at the moment so if anyone has the dimensions, I'll buy one at lunchtime just in case. The new filter is from Halfords if anyone knows if they include a washer too.