ATMEGA 32A PU signature
Page 250 (section 26.3) of the full ATmega32A datasheet:



The "PU" designation is a suffix for ordering purposes, detailed on page 320. The "PU" just refers to the 40P6 package, which in turn is a 40-pin through hole version.

If you compare the ATmega32A to the ATmega32 (without the "A"), the signature is the same. The "A" revision is a replacement version with power consumption optimizations. Atmel has a guide specifically for migrating from the ATmega32 to the ATmega32A.