There are two issues revolving around pterosaur fingers.
1. How many are they? Others say 4. I say 5, and #5 is a vestige.
2. What was their orientation? Others say palmar side anterior in flight. I say palmar side ventral in flight.
Manual digit 5
This specimen, WDC CSG 255, is yet one more example of many demonstrating the existence of a vestigial manual digit 5 on the dorsolateral corner of metacarpal 4. This configuration reflects the primitive lateral placement of manual digit 5 plus the enlargement and axial rotation of metacarpal 4. On the left hand manual digit 5 is buried beneath metacarpal 4.
Digits 1-3 Orientation
Earlier we discussed the drawbridge effect on manual digits 1-3. Each metacarpal is bound to its partner laterally and metacarpal 3 is bound to metacarpal 4 laterally. Since metacarpal 4 is enlarged and axially rotated this 3-4 connection is low, on the medial (now ventral) side of metacarpal 4. This observation is confirmed again in WDC CSG 255, in which right metacarpals 1-3 rotate up against right metacarpal 4, but note they are not bound to it. The 3-4 hinge is broken yet metacarpal 3 is partly ventral to metacarpal 4. The left hand is more natural in configuration with metacarpals 1-3 exposed dorsally and in the plane of the wing while metacarpal 4 exposed laterally (axially rotated to dorsally). Note that the base of metacarpal 3 is largely tucked under (ventral to) metacarpal 4 as in the left metacarpus. Metacarpals 1-3 are not bound dorsally to metacarpal 4 in either case. Rather metacarpal 3 is between 2 and 4, as in other tetrapods.
Frey E, Tischlinger H 2012. The Late Jurassic Pterosaur Rhamphorhynchus, a Frequent Victim of the Ganoid Fish Aspidorhynchus? PLoS ONE 7(3): e31945. doi:10.1371/journal.pone.0031945