At the beginning as Ζπ 2850Χ-28755. Then as Z1 21 73 107 7 100-316.... only 217 of them got the U.I.C. / R.I.V. numberings as soon as the others were destroyed, shanged use e.t.c.
They were built, as Christo said, by many builders between 1946 and 1948. The builders i have found are these three:

CHAS ROBERTS & CL. BUILDERS WAKEFIELD, ENGLAND, 1947. The most common builder....

METROPOLITAN CAMELL CARRIAGE & WAGON Co. Ld. BIRMINGHAM, 1946

HURST NELSON & CO. LTD BUILDERS, MOTHERWELL, 1946 and 1948.
But three more builders have been reported...:

CRAVENS SHEFFIELD, 1946

GLOUCESTER, 1946

SMETHWICK, 1946
At the beginning at least some of them had plywood instead of the usual planks.... This one is the only one still having the plywood, at Gefira.
21 73 107 7 103-5 - Lachanokipi
21 73 107 7 138-1 - Kilkis
21 73 107 7 142-3 - Lefkothea
21 73 107 7 159-7 - Lianokladi as part of the 10th movable maintenance train of the mechanical facilities, of the T.M.K. (Sector of the Metalic Constructions).
21 73 107 7 210-8 - Nea Magnisia
and one more at Polisitos
Finally this one is converted to a storeroom at Rodopoli's station.....