Upgrading to hard pipes

I've ordered most things I need and realised I haven't got the 'o' rings. I'm a complete novice when it comes to this so I'm in need of advice on to what size I need. I got 50-51mm and a 60-51mm couplers from creationmotorsport_uk on eBay as recommended on previous posts.

171266960379 that's the item number for a pic of the part. I'm on my phone and useless at adding pics sorry.

I know I'd have to get 50mm and 60mm but don't understand the other sizes it's coming up with like thickness...

Hope someone can help

Thanks