I recently came across an 8.5 GM 28 spline posi unit from a 1974 Nova. The fellow said it was a 3 series as it originally had 3.23 gears. Is there any way to tell if it is a 2 or 3 series? I don't want to use a spacer for 3.42 gears. Thanks.

The 2 series has the casting number 410409 and the 3 series has the number 410408.

BGH, Thanks for the info. I confirmed it was a 3 series posi.