Wait a minute. I understand not wanting to order two parts just to do a comparison, but you already have both in your possesion. A big block WP is held on by what? four bolts? Yank both of them and do a side by side comparo.

I betcha its just a matter of fin count as already mentioned. I'd also be willing to bet that even though there are two different part numbers, the older one has been superceeded by the newer one.