Posting/mailing letters and cards early-mid 20th Century UK
What's in a name: post or mail
The word 'mail' is a comparatively recent introduction into the English language. While I was growing up in 1940s and 1950s Britain, we 'posted' letters and cards, we didn't 'mail' them, and the postman, not the mailman delivered the 'post' not the 'mail'.
Postal collections and catching the post
There were several collections a day and they were extremely reliable. What was more, we always knew whether or not we had caught the post at a post box because the there was a small white enamel tablet which stated when the next post would be. It was changed at every collection. You can see this in the first photo below. The updated tablet could be slotted in every time that that the door was open for collection. When the door was closed, the bar above prevented anyone unorthorised from tampering with the tablet.
Late postal collections in the mid-20th century
While I was away at university in the 1950s, I could cycle to Exeter St Davids station and post letters and cards home to Cambridge as late as 11.00 at night. The sorting was on the train and everything always arrived the next morning. There was no extra charge for this - and there was no first or second class.
Letter boxes /post boxes /pillar boxes
Provided that we had the appropriate postage stamp, we posted letters, ready-stamped, in what were variously called post boxes, letter boxes and pillar boxes. For larger items like packages and parcels, we had to go to the post office for the item to be weighed and costed.
I suppose that 'letter box' and 'post box' were general names and that 'pillar box' was originally for freestanding column-style boxes, but people just seemed to use whatever term came first to mind. The term 'mail box' was never used.
Post boxes were always red and showed the name of the monarch at the time of installation, but as I was never interested in this at the time, I took little notice. In view of the Second World War and the austerity afterwards, I doubt if there were many new post boxes when I was a child. Victorian post boxes were even still in use.
Identification of Geogian post boxes
The Georgian post boxes in the above pictures have no numeral, so must be from the time of George the Fifth. Before him there had not been a George for 80 years, which was before the establishment of the Penny Post in 1840. Note that the Georgian post box in the last picture does not have the small white tablet for details of collection times, etc.