repeat bar at the beginning of a piece

[Steve Tarr]

I'm working with 2.13.20, trying to reproduce a piece of shape-note 
music that has a 4-dot (with no line) repeat bar at the beginning of the 
piece.

The snippet titled "Printing a repeat sign at the beginning of a piece" 
works as described if I use the "|:" bar, but doesn't display the bar if 
I use ":" (or any other bar style) instead.

What's special about "|:" in this case?  Is there some property I can
override to allow arbitrary bar lines to be displayed at the beginning
of a piece?  Or is there something I can change so that ":" will be 
treated as a repeat bar just as "|:" is?

-Steve Tarr