Confused

[nsb29]

I ordered new gears for my 65 but when I went to install the carrier it didn’t fit, after a lot of Ass scratching I find the ring gear is 1/4 inch thicker than the old one. Summit told me I ordered the wrong one but in reality I ordered the gears for a 65C 10 1/2 ton they are the ones that determined the gear they sent me, in my research I understand that the difference is there is a series 3 and a series 4. I don’t understand why they didn’t ask me which differential I had instead of just picking one and sending it to me. my question is can anyone shed any light on what determines the two different housings and were they both available in a 65 1/2 ton. Now I have a problem with returning it as I have installed it,
so I’m looking for as much information as I can hoping to come up with a good argument as it is their mistake
Thanks for any help I can get