Traditional catalogues aren't going to give a number for this item. You might know that postal stationery, including wrappers, are found in the Higgins & Gage catalogue. Vol. 17, in the case of Trinidad and Tobago. These catalogues haven't been updated in decades, but you'll still see HG numbers used in auction listings.
Robson Lowe's Encyclopedia covers much of the Empire/Commonwealth, and includes some postal stationery, but it doesn't cover Trinidad and Tobago.
As always, corrections or clarifications are invited.