A long time ago, though it was a WRT54G (same case design). Linksys was going through multiple iterations of the electronics inside, at times with no indication in the model number that anything changed. It's best to check your unit yourself to be certain.

iFixit has some community guides on repairing/opening various Linksys units here: https://www.ifixit.com/Device/Linksys_Router

Basically that blue plastic piece is snapped into the black plastic piece. Refer to the pictures in these two teardowns for help on where to squeeze to open the case:

https://www.ifixit.com/Guide/Linksys+WRT54GS+v2+Rear+Case+Replacement/7126
https://www.ifixit.com/Guide/Linksys+WRT54G+Motherboard+Replacement/7017