Hey, the Round shaped release is just that, round, it is a plastic pin with stippling molded onto either end. It has the same wedge shape molded into it to actuate the release lever in the frame as the D shaped. The D shaped mag release was first used on the SR9c. The SR40 got it and when Ruger started producing the SR9 again after they stocked up on the SR40 the SR9 got the updated mag release and take down pin.
As for the internals Josh is right. At the release of the SR9c Ruger improved the internals of fire control system to create the improved trigger most like in the SR9c, and later in the SR40. Sadly the SR9 did not get the improvements till well into the production of the 331-xxxxx SN units. Not all 331 SN will have the D shaped release but they all seem to have the updated fire control parts. Ruger changed several components in the update. I sell a update kit on my site that includes all factory parts to do the update yourself. Will be cheaper to get it than to pay shipping and FFL fees to return it to the factory, in most cases.
Anyways hope that helps, I suggest don't worry to much about it, clean it really well, in the striker tube and channel then shoot the snot out of it and enjoy it!