This is getting old fast !!!

[Bowed]

For the second time I took out the 3 speed standard transmission in the 75 C10 that I got recently , trying to find that mysterious knocking noise that comes from the bell housing area ( It had the noise when I bought the truck ) and while looking thru the repair manuals I noticed the bolt pattern did not match any thing in the manuals . It has a 5 bolt pattern and all the transmissions in my manuals are 4 bolt pattern so now I wonder if my problem is that some body put a mis fit in there . The trans is a GM21 ( 352259 ) ( H30 -4 ) can any body tell me if this is for 75 chevy ?