Westerfield made this car and still lists decals.

  D8101 DECAL, O-27/O-27A MILL GONDOLA, B&O   $5.50 
Note also that the drop-end was modified by the B&O on the O-27a series. Not easy to describe, but a side-by-side comparison makes it obvious. I haven't looked at the Walthers car in awhile, but it maybe more suitable as an original USRA mill gondola, B&O class O-27.

Here are my notes mostly from the Westerfield History and Instructions:

B&O received 500 USRA mill gondolas from Standard Steel Car Co. in 1919.  They were given series 250000-250499 and classed O-27. Like NYC and Reading, B&O adopted the USRA design with minor changes as its standard throughout the 1920's.  A total of 6,000 clones were purchased between 1922 and 1925.  While all were classed O-27A, three of the five purchases differed physically from one another and from the USRA design.  The 1922 cars built by Cambria differed only in the removal of poling pockets and the use of Washburn/Tatum-design arch bar trucks.  Beginning with 1923 Bethlehem Steel took over the Cambria plant and continued filling orders for the gondolas.  On 1923 deliveries the corner was strengthened with a pressed steel post which allowed the end opening to be expanded from 7'11" on earlier cars to 9'0".  There was also a switch from Carmer cut levers to bent rod style.  The trucks on the early 1923 deliveries were Washburn/Tatums but beginning at about car 252400 they were switched to Symingons.  With the 1925 deliveries the drop ends were changed to a style uniquely B&O.  A single large, inward "crease" ran the width of the end.  The stake hold-downs were revised to three links of chain secured at both ends; the tack boards were placed in a higher location and standard 70-ton cast side-frame trucks were used.

Regards,

Bob Witt