When a UK citizen renews his passport outwith the UK, the new passport shows 'FCO' as the place of renewal instead of the visa office that did this. In China, this causes the local PSB to doubt said passport, so they ask me what does FCO mean. I don't tell them that it means 'For Crown Office'. Instead, I leave them to wallow in their ignorance. The first time I told them it means 'Ulanbator', from where I returned with a temporary UK passport.

I always make a point when renewing my L visa of only letting them see my current 'FCO' passport. They notice there's no entry stamp in it - alarm bells! Then I show them the 'UB' temporary passport that says 'FCO', which they distrust too coz it was temporary! But it has the required entry stamp.

So, I have to submit 2 passports to get my new visa.

I previously had a Brit 'ordinary' passport issued in Beijing, that didn't specify the place of issue as 'FCO', but as Peking. More trust then!
Indeed, the PSB here are an absolute joke. It takes me all my time to stop laughing.