Stanwell
Most references state that there should be a "dash" between the type letter and the model number but what they show as an example features a hyphen i.e. no space before or after. Having said that, you will find instances where the type letter and the model number are stencilled on the aircraft without dash, hyphen or space. Here's one example from a USAAF manual of the period. This shows it with a dash i.e. P - 38 but this doesn't look right so most authors usually settle for a hyphen i.e. P-38
The less said about "Airplane" the better.
Aroa is spot on about the British version of the P-39 Airacobra being the P-400. They also messed up the P-38 which became known as the P-322.
Centaurus, apologies for hijacking your thread with what might seem trivial to some. None of this detracts from your usual entertaining style.
Stanwell, many thanks for your kind offer of some apostrophes. I've got a whole bunch of plural's just crying out for an apostroaf so your offer is gratefully accepted
Rgds