Auto Number based on Value from another table

The issue here is how to rank by Record ID#.  There is a great video here by The Quickbase Junkie. https://www.youtube.com/watch?v=sHU8pB38N2Y

Not tested but the Formula Query (a numeric formula field) to rank by Record ID# would be

var text Query = 
  "{3.LTE." & ToText([Record ID#]) & "}"

Size(GetRecords($Query))

That should return a count starting at 1 to sequence the work orders. You could use that as a suffix instead of A B or C or else you would have to use a formula to translate from the ranking number to the A B C letter of the alphabet.