An image search on "playmobil 7923" shows the lightning bolt more commonly, and the swirly one occasionally - but the latter, only in stock photos. I searched through my site-scrapes from that time, and see 7923 pictures with the swirly design in 2008 and 2009 (in various countries), but lightning bolts thereafter; and
in the catalogues we know about with 7923 in them, 2007 shows the swirly one and 2008 and 2009 show lightning bolts. In short, I think the swirly one is a discarded prototype. (Shame - more visually homogeneous with the oval shields.) cheng's part number is also likely to be right: I have only two round shields of unknown origin, and 30 63 9152 occurs "near" part numbers first appearing in 2007, whereas the other, 30 64 7982, occurs "near" parts first appearing in 2010.
Anyone else who ever ordered this part, please chime in - but that's good enough info for me! Tim, you may want to update your shields page. I found the part number for
this Indian shield too - 30 64 0420, came with Figures Series 9 Boys.