A google search with that terms : Eilzugwagen AB4ys Zeichnung gave me that link :
"Wie immer", it's in german language, so it's not so easy to choose the good one, but I think :
-the 3 first drawing show the wagon in early 1930. years
-the 3 next one show the wagon in 1955 after they were modified...
-the 2 next one show the "welded" version (geschweißte Ausführung ca. 1934 und 1936). It's seems to me that we are not far from....But the "double door" is only at the front of the wagon
-and the last drawing shows a light version of the Welded one (Geschweißte Leichtbauausführung)
with double doors at the both side, so may be we have caught the good one,
but the guy says " nicht zu verwechseln mit den echten Schürzenwagen :B4y42a....",
you have to understand : Do not confuse with the real wagon B4Y42A....
So, we are not far from the good one, for me, I think I can live with this version but maybe some other chearch will give us the real solution.....
You have to be registered to see more or bigger pictures
A last thing, don't forget also that they are wagons for travellers, but for a "Lazarett Zug" the inside will be completly different...
and an other link :