Dave,
I've had a few early Motown pieces with thick black marker on like that - but more of a NR than DR (I always assumed it meant No Returns).
Usually they have been Styrene and on the Globes Tamla label - the difference in the letters could be down to someone rushing the job, or even a couple of different people in the returns department being sat marking them - lets be honest, sitting and marking the returns by hand must have been a right ballache.
I reckon they did them by hand as Styrene cracks easy when drilled (ABC did the same with Gold Paint Stars / Blobs).