Malaysia-Singapore Joint Issue - Birds

Pos Malaysia and Singapore Post released a joint stamp issue on June 27, 2002 on topic of birds. The stamp design of both the postal authorities was similar. Four different bird species depicted on this fine stamp set, i.e. white-bellied black woodpecker (30s, black-naped oriole (30s); Shelly's sunbird (RM1) and blue-backed fairy bluebird (RM1). The stamps were issued in setenant pair format in a sheet containing 20 stamps. I noted that some stamp sheets (not all) have serial number on the left margin of the sheet (see the image below).