The mailboxes, where I grew up in Pennsylvania, tended to look like this:
So, imagine my surprise and nerdy thrill of discovering the post boxes in the UK!
Most are red, but depending on the year when they were placed, had an insignia for the king or queen on the throne at the time. The initials are called the "Royal Cypher" and are usually a monogram of the monarch. More information can be found at the Postal Museum website , should you want to learn more.
Here are a few photos of the ones we have discovered so far.
V.R. (Victoria Regina), reigned 1837 - 1901
G.R. (George Rex V), reigned 1910 - 1936.
G.VI R. (George Rex VI), reigned 1936 - 1952
E.II R. (Elizabeth II Regina), our current Queen Elizabeth II, reigned from 1952 - current
Still missing from our "collection", are the post boxes from the Edwardian reigns (Edward VII - 1901 - 1910 and Edward VIII - 1936).
When a new monarch took the throne, old post boxes weren't replaced, new ones were just added, so there are a variety still existing around Britain. Notably, since 1953, boxes made for Scotland have the Scottish Crown, and not the E II R cypher. From the postal website, this is "because some Scottish people do not accept the current Queen Elizabeth as the second monarch of her name, since Queen Elizabeth I was never ruler of Scotland."
There is still so much to learn.