You're not buying a real IBM cable, those are absurdly expensive. You're buying a knockoff, which is probably why the part no is different.
If the cord isn't compatible, you should be able to return it for a refund.
I've bought a dozen off brand chargers for phones, laptops and other electronics on eBay. They've all worked just fine.
The only thing I shy away from are rubber cases for phones, they tend to be garbage.
My lenovo/IBM z60m replacement charger works great. Compard to the original, the cord isn't quite as long and the plug isn't quite as snug in the transistor (big black box) but it works just fine.
In general, monoprice.com and/or meritline.com are the place to shop for cheap cords, but I've only shopped for video, audio, computer, networking cables there (never buy from best buy, who marks up cables about 10-100x their original cost. The 3 hdmi cables for 5-10 bucks work just as well as those outrageous 99 dollar monster cables)
I'm not sure if they have good deals on cables like these though.