The reference is to the Palazzo Agnusdio / Agnus Dei at Santa Croce 2060, Nadali & Vianello (1999) Tav.16. It has a five light window on the facade decorated with emblems of the four Evangelists:
The door described at Notebook M p.215 is shown below:
There are references at Notebook M p.102, Notebook M pp.215-6; Bit Book p.31 and House Book 2 p.5.
For further images and comments see here and the links provided there.
For the Monday morning referred to at Notebook M p.102 see the first letter, dated December 23rd, of the two letters of this period from Ruskin to Rawdon Brown.
[Version 0.05: May 2008]