Posterior of 8.6 mm specimen, and silhouettes of three species based on figures in Kaas & Belle (1985).
X: position of mucro.
L. cancellatus: “Tail valve …. mucro behind the centre when seen from above, swollen but not prominent, posterior slope a little concave directly behind the mucro.”
L. sarsi: “Tail valve …. mucro not prominent, central or a little anterior, posterior slope only slightly concave directly behind the mucro.”
L. scabridus: “Tail valve …. mucro about central, somewhat swollen, postmucronal slope concave.” (Kaas & Belle, 1985).

The photo matches best the L. cancellatus K&B description and silhouette, being entirely concave behind the mucro while L. sarsi changes from concave to convex halfway down the slope.
Note that the position, measured horizontally, of the mucro on the photo appears to be anterior of the centre, but this is because part of the anterior is concealed below the next valve. The hidden part can be seen as a white, unstained, anterior border on the extracted valve in image a 06 Leptochiton cancellatus . So mucro position is not a reliable diagnostic feature on living specimens.
The K&B text and silhouette of L. scabridus are inconsistent as text says mucro is about central but on silhouette it is well to posterior. The absence of the diagnostic bright red foot on the photographed specimen showed it was not L. scabridus.

KEY ID FEATURES and similar species at: 02 Leptochiton cancellatus