I guess the bearing size will be 39 x 72 x 37 mm.
My guesses are based on the following:
With the casting number mentioned in the drums of my trailer (564829
) I've found 2 possible references refs. in AL KO's catalogue (pictures in post #7):
EURO PLUS DRUM 578528
140x5/M14x1.5 TYPE 2051Ab CASTING NR. 564829 ETI NR. 811158
EURO COMPACT DRUM 578825
140x5/M14x1.5 TYPE 2051Ab CASTING NR. 564829 ETI NR. 811268
I don't know what is the ETI nr. or what it means, but while checking the 2 only bearings available and compatible with the 2051 brake type:
34x64x37 ROULEMENT CARTOUCHE 34 X 64 X 37 - www.pieces-alko.fr
39x72x37: ROULEMENT CARTOUCHE 39 X 72 X 37 - www.pieces-alko.fr
I found that the bearing 39x72x37 is compatible with both of the above ETI nrs. wilts the 34x64x37 is compatible with one of them only:
ETI NRS. COMPATIBLE WITH 39x72x37 BEARING:
ETI NRS. COMPATIBLE WITH 34x64x37 BEARING:
Nevertheless, before ordering the parts I will ask my brother to disassemble the drum and reconfirm which one is it.