O2 sensor question

98 Grand AM GT with 3.1 auto - I need to replace the O2 sensor before the cat BUT both parts stores I have checked with have a sensor with 4 connectors but in a square plug - mine is a flat plug - any ideas what is up? I just aquired the car through a trade so know nothing about them.

Thanks in advance for any information.

Its a 96 or older?

Whats the 10th digit of the VIN? T=1996, V=1997, and W=1998

The 10th is a W so it is a 98 BUT I have good reason to believe (PS pump only on with 1 loose bolt - exhaust not tightned up - sheared wire on O2 sensor) to believe that the engine has been replaced. The engine could be from a 96 or something. Would that explain the plug difference? Thanks!

Engine is a 96 - thanks!