Is this narrow gauge? If so, the D&RG was using a non-MCB collarless axle (called a "Muley") on their standard freight truck. This bearing has a face to bear on the end of the axle to control thrust.
If standard gauge, the original MCB bearing was 3 3/4 x 7... This became the MCB A bearing when the B size (4 1/2x8) was adopted in the mid 1880's, followed by the C (5x9) in the 1890's...
MCB bearings A are very hard to find... B's and C's are out there...
