Air Letters, Sea Letters, Air Mail forms and 'Airgraphs'

 What is the difference between 'air letters', 'air graphs' and the air mail forms?

It seems that the 'air letters' are ordinary letters sent by air mail (and presumably charged by weight).  Increasingly as the weeks go by, more and more of Stan and Grace's letters are written on air mail forms such as those shown in this photograph.

The first 'air graphs' between Stan and Grace make an appearance on 6 April and look like a small photocopied version of a brief letter written on a special airgraph form.  North Lincolnshire Museum's web site tells us that airgraphs were invented in the 1930's by Kodak Eastman and a service was set up in 1941 to transmit airgraphs between the Middle East and the UK.  (northlincolnshiremuseum.co.uk/discover/curators-choice-airgraphs/).  The airgraph forms were photographed on mircofilm in Cairo and the film flown back to the UK.  This took up less precious space than paper letters on the aircraft flying between the UK and Cairo.  The messages were then printed out in the UK and delivered.

The Victoria Museums, Australia, web site (https://collections.museumsvictoria.com.au/articles/16591) shows a newspaper article with photographs of the process.

'Sea letters' are usually much longer letters, often with postcards or other things enclosed and, coming by sea, take much longer to arrive.