That makes a lot more sense of your last post on the Drummond Group. The Drummond spindle was at best drilled 3/8 and the Myford ML4 19/32 so with your reported about 1/2 bore it looks like yours was indeed bored out thus damaging the end of the thread.
You can now check your serial (far right of bed) against the list on the Lathes.co.uk site to find the year.