I got the year messed up too. That screw, the one with the pointy end (I just love this high-tech jargon) 55981-S2 (vs 55891-S2, OZ) has the same dimensions as 31497-S2 which is used on 64 thru 68 (mostly 64-66) with both the diamond head and "D" head marking (although the "D" may be more clear zinc, -S8, finished uses). As both 31497-S2 and 55981-S2 have the diamond head marking (same manufacturer), there may some other difference, like grade (although not marked), undercut head (aka UBS style) or a manufacturing process specification difference. From 1968 thru 1973, 55981-S36 (and -S36M) were used for most of the underhood grounds, brake lines, etc.
Looking at Jeff's pictures, the head seems to not be as tall as either 31497-Sx or 559841-Sx that I have.
Jim