UB registrations were issued from October 1929 until April 1932
JB registrations were issued from March 1932 until August 1936
Neither really tallies with the description "late 30's" but JB would be nearer.
For the record the UL mark can be more accurately dated as between Jan and March 1929, London's issue rate being considerably greater than both Reading and Leeds.
As Dave says, none of those registrations are now active in either the UK or Ireland.
~iw
