Details for the LPC11U37 Stamp are here
A details pin list, as defined in the #include "STAMP_PINS.bas"
Pin 1 of the Stamp form factor is SOUT (serial out) and pin 2 is SIN (serial in). These serial pins are TTL level and positive true. Pin 3 is the ATN pin and in the ARM Stamp case when held low during RESET or power on, the Stamp will appear to the PC as a disk drive that can accept an mbed binary images.
The open source schematic of the Stamp below
The heart of the ARM Stamp is NXP's LPC11U37. It is a 5V tolerant part, and details from NXP here.
If you want to go beyond simply changing IO pins, details on the dedicated hardware peripherals here.
BASIC allows direct access to hardware peripheral through *pointers. This is equivalent to peek and poke in early versions of Microsoft BASIC.