757-200et?

Have just been looking through some pics of the web and came across a BA 757.

757-236ET. Whats the ET bit for? I have had a discussion with Mark L about it, but neither of us really have a clue.

Its was on G-BPEC http://www.airliners.net/open.file/286958/M/

Though Mark said he found it on another BA aircraft.

Anyone with any info, that would be great. Cheers. 🙂