I found this image on the Pitcairn Islands Philatelic Bureau website
http://www.stamps.gov.pn/I must say, it isn't the format I expected. Either way, I don't think it qualifies as
tete-beche, does it? The adjacent stamps would have to be oriented 180-degrees in relation to one another, wouldn't they?

Technical Details
Release Date : 1 December 2002
Denominations : 40c, $1.00, $1.50, $3.00
Stamp Design : Donna McKenna, Wellington, NZ
Printer : Joh Enschede, The Netherlands
Process : Offset Lithography
Paper : Sopal Stimat 110gsm
Stamp Size : 38.89mm x 38.89mm triangular
Perforation Gauge : 13 x 13.25
Sheet : 50 triangular stamps per sheet
First Day Cover : $6.90
KirkS