N(delta)-acetyl... vs N(alpha)-acetyl
The difference does not lie in stereochemistry. Rather, N$^\alpha$-acetylornithine and N$^\delta$-acetylornithine are regioisomers of one another.

The superscript Greek letter denotes which nitrogen is substituted with an acetyl group. The lettering is relative to the carboxyl group as shown in the image below:



Note that:


The carboxyl group’s carbon does not get the $\alpha$, because it is considered part as the functional group from with the labelling begins
The $\alpha$ carbon is asymmetric. Both L- and D-compounds (or (S) and (R) according to CIP nomenclature) are possible for both N$^\alpha$- and N$^\delta$-acetylornithine.