WHY ............does anybody know the answer

Ok, I am into a total rebuild on a 1966 Valiant. The engine is a slant 6. I bought some main bearings for it. STD. I bought Michigan brand because I have used them before no problem. The last /6 I used these bearings and that engine runs fine. Didnt pay any attention to this. This engine, I opened up the box and started to put the thrust bearing in and it wouldnt fit. Found out that they slipped a cast crank bearing in the box. so I started looking at the rest of the boxes. The box says : consists of 1 pair 7376T, 1 pair of 7377T and 2 pair of 7378 T. The 7377 T is the thrust bearing. so, why do they have 2 different numbers for the other 3 journals. Arent the other 3 journals the same. I checked the bearings and there is no difference in them. they all fit good and look the same. But I am stumped as to why there are 2 pairs with 1 number and 1 pair with a different number. I have checked other sets of main bearings and they have 1 thrust bearing and 3 sets of regular bearings. So, does anybody know why. I am kinda leary now about using these. What kind of bearings do you use on a forged crank for '66. I need to buy a couple of sets as I have another '66 engine after this one.

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