Agree, Email receipt for ticket purchased March 18 has NO ticket number.

Comparable document for another booking puchased today does include the ticket number.

Further on this, if you look your booking up on line and select the "View E-receipt" option, the pop up window contains the 016 ticket number.

You can note this down or use the Email receipt option from the pop-up to send a copy the receipt to yourself. This contains the ticket number as well.

Thanks for all the responses, I got the receipt using the suggestions here!

Glad I found these forums to get better informed, particularly given my experience calling the United customer service desk!