A rare Southern Rhodesia 6d black and mauve was the undoubted star of part one of the Arnhold Collection stamp sale on May 17.
The horizontal pair from the upper right corner of the sheet, featuring an error imperforate, sold for £28,000 at the UK auction.
The item's great rarity boosted it 16.6% beyond its £24,000 high estimate; it is one of only six possible pairs, and is unique in this position - another of the six resides in the Royal Philatelic Collection.
Light gum creasing and a tone spot in the right selvedge were the only detractions for this brilliant-colour specimen.
German-born Hans Eduard Arnhold died in 2011, aged 89, leaving behind a lifetime's work in stamp collecting, including several superb items from British Africa, which formed this auction.
The sale also featured a Sierra Leone 2½d on 2s dull lilac, type 10 surcharge, which sold for £13,000, comfortably surpassing its £12,000 high-estimate.
Just 16 examples of the stamp were produced.
Head of auctions Richard Watkins commented: "The hard work put in by the auction team has been well rewarded with a total sell out and a wonderful total realisation of £900,000, far above pre-sale expectations."
These four Great British investment-grade postage stamps at PFC Auctions' online sale are currently available for bidding until May 24.
The leading 30 collectible British stamps have risen in value by 10.8% pa since 1998, according to the GB30 Rarities Index.
We will be bringing you a full preview of the second part of the Arnhold Collection sale, which is set for September. In the meantime, make sure you keep abreast of the all the key stamps news by signing up to our free weekly newsletter.