Correct fitting of diaphragm kit for Baxi 105e combi boiler?

Ordered and fitted a DHW diapragm kit for my Baxi 105e combi-boiler. Only slight worry is that I have a tiny o-ring left over in the kit and couldn't see where it was supposed to fit?

I could see that the washer in the kit went over the new brass nut..... but puzzled by the o-ring.

The system appears to be working now, but I'd hate to spring a leak in the near future for the sake of an o-ring.

Guidance appreciated!