The Museum operated two Post Offices and two stalls to sell these items and also had a supply of "plain blue" cards. These 'blue' cards were to be sold for 1d and would received an impression of the special exhibition cancellation. A fee was charged for handback cancels on cards, etc. brought in by the attendee.
The above is a specimen of the Kensington envelope. It is unknown how many were so stamped or if any insert cards were also marked. In his exhibit, John Davies reports three 'SPECIMEN' examples are known to him. As this example is of unknown provenance, it could be one of those previously reported or a fourth.