The stamp in this thread is a pretty early Plate 1L impression.
The B relief characteristics, especially on this position show up a lot better on later impressions. Particularly at the top of the colorless oval, the break becomes clearer later on.
The link below shows a good comparison of earlier vs later impressions - the b/w chase photo on the left being earlier, and the stamp on the right being later, and showing the top oval break better.
http://stampplating.com/stamps/448