20+C+M+B+10

20+Caspar+Melchior+Balthasar+10
20+Christus+mansionem+benedicat+10

A tradition associated with Epiphany in southern Bohemia of the baroque period, at least:  children collecting money for the poor inscribe C+M+B plus the date in chalk on the lintels of the doors that are opened to them.  Bernard Klasen, "Passer la porte, penser la porte," Transversalités (Institut Catholique de Paris) 92 (Octobre-Décembre 2004):  123-124 (123-130).

Martin Walraff's "«Ego sum ostium»:  Kirchenportale und andere Türen im antiken Christentum" (Theologische Zeitschrift 62, no. 2 (2006):  321-327) has more on this at pp. 324-325 and 325n16 (where he gives three further references), though without restricting the custom to southern Bohemia.  The example he gives is 20+C+M+B+10.