Chris,
From the mid twenties till the T35, Douglas used a deep track thrust bearing on the top of the steering head (not all models). That bearing consisted of 2 identical halves (part number 2016) and loose 3/16" balls. Unfortunately that is an imperial size bearing that is no longer generally available - the O.D. you quoted (1.717") is actually 1 21/32", so it matches. Whilst bearing stockists no longer keep them, you may be able to get them from LDMCC Spares - at one time, they were getting them remade for the T35.
Regards,
Eddie.