A Trinidad Imperf. Forgery
At first glance this looks like a very fine example of a 4d Britannia Imperforate of 1859, which catalogues 275 pounds in Stanley Gibbons. But under careful examination it proves to be a forgery. The forger used a later stamp, most probably a poor example of a 4d lilac pin perf. This was precisely cut down to the edge of the design and the back of the paper sanded to make it thin. Afterward this was glued to a slightly bigger piece of white paper to give the illusion of wide margins, and the numeral cancel was extended to cover the new margins.
It pays to remember the maxim “Let the buyer beware”!