I believe Ps.-Mk refers to the book Pseudo-Mark aka Acts of Barnabas but is tagged as if it were the Gospel of Mark consistently in Allison, Dale C., Jr. A Critical and Exegetical Commentary on The Epistle of James. Edited by G. I. Davies and C. M. Tuckett. International Critical Commentary. New York; London; New Delhi; Sydney: Bloomsbury, 2013 and perhaps other works.