Camlock sizing?

[les_danger]

I think i'm going mad. Got an external letterbox in the porch. Previous tennants left the camlock in the locked position and didn't hand over the key (they lived down the street for about a year before i got here, so I expect they wanted to continue getting their post like this).

Now switching over the camlock was supposed to be a 3min job on day 1. But I can't seem to find anything that fits. I'm sure the cost of replacing the whole thing is cheap, but i'll be buggered if i can't just replace the lock. The opening is a double-D shape, with minimum width across the flats 11-12mm. Face plate is round and 20mm diameter.

Everything i've tried or found online seems too wide to fit into the opening. The main measurement when googling seems to be the length of the body rather than the width. In this case the length doesn't really matter so long as it's short enough, the horseshoe clip just binds it on the rear. I would have thought these are really common, am I googling the wrong thing ('camlock' ?) Sorry photos are a bit wonky, any help much appreciated.

Opening

 

[foxhole]

Measure diameter not across flat sections.
Unclear from photo .

Cam Lock for Door Locker Cabinet Drawer Cupboard Mailbox 16mm 20mm 30mm UK | eBay

High-quality Lock and two Lock keys. Keys are removable in Locked & Unlocked Position! Ideal for desk drawers, glass doors & lockers. If item defective after 3 month, you can still send it back to us.

www.ebay.co.uk

 

[les_danger]

Sorry here are some better pics...

Face at 20mm diameter

15mm on the widest part of the barrel

13mm on the flats

Total end to end length of the barrel is 18mm

Opening is 16mm at the widest

and 13mm at the narrowest

 

[les_danger]

Looks like a Sterling one, can find a close fit but it doesn't give details on the narrowest part of the barrel just 15.3mm width.