|Schrader spark plug air pump advert|
The 1974 'a guide to Land Rover expeditions' has a few interesting paragraphs on tyres, as the quote above indicates. Metal dust caps are still available (check your local ebay) however I don't think I know anyone who has metal dust caps fitted!
One item in particular caught my attention: 'Spark plug air pump (Petrol models only)'
The paragraph continues: 'The latter is extremely useful, as manually inflating large tyres in hot conditions can be very exhausting'.