Origin of term meter maid?

Standing by a parking meter/ When I caught a glimpse of Rita

The use of 'meter maid' first appeared in the US in the late 1950s. It caught on after several cities began to introduce teams of dedicated 'traffic wardens' to deal solely with motoring offences.

The term was still largely unknown in the UK when The Beatles released Sgt. Pepper on 1 June, 1967.

Where did Paul McCartney get the phrase from?

  { I read} that in America they call traffic wardens ‘meter maids I thought, God, that’s so American! There’s something good about ‘maid,’ and ‘meter’ made it a bit more official, like the meter in a cab; the meter is running, meter maid. Hearing that amused me.  

Taken from Many Years From Now

Paul has also linked Lovely Rita to receiving a parking ticket from a young woman when returning to his car. The exact circumstances are disputed, see here

Sexual suggestion

McCartney also hinted that he used the word 'maid’ because it had sexual connotations, like a French maid or a milkmaid.

Or perhaps meter maid was easier to rhyme than parking attendant!  

Paul tries to charm his way out of a parking fine.

Was there a real Rita?