The Mulready illustration was printed on the sheet such that it appeared on the face of the envelope.(As a point of interest : all mechanical printing devices from the Gutenberg press on are primarily designed to process flat rectangular sheets. Hence the illustration would have been printed using a press and then cut to a diamond shape.The number produced from any one sheet naturally depended on the size of the printing bed and to this day envelope printing and envelope manufacture have maintained a symbiotic relationship.)
In a sense the Mulready illustration was a very elaborate frank indicating that postage had been paid.
There was nothing to stop you writing on the inside. Consequently the Mulready wrapper was fundamentally akin to the present-day aerogramme.
However, the Mulready envelope suffered an inglorious demise. The design showed a munificent Britannia bestowing the benefits of postal services to the countries of the world under British control.
The design was so much over the top that it generated widespread ridicule and lampooning.In addition it was perceived in some areas as a covert government attempt to control the supply of envelopes and hence control the flow of information carried by the postal service (which had become solely a government monopoly under the reforms). As a result of the uproar the wrappers were withdrawn and a machine was designed and built to destroy the stocks.