bug using doublebox in tbl with ditroff

[Robert N. Berlinger]

Does anybody know of a bug in ditroff which causes the box 
produced by the doublebox option of tbl not to line up properly?  
The boxes don't quite come into agreement at the edges.  I'm 
working with a QMS 800 and QUICroff software.  I have contacted 
QMS and they seem to think (and I agree at first glance) that 
ditroff is probably at fault.  Anyone who could give me a lead 
would be greatly appreciated.  Thanks...  
-- 
Robert Berlinger
...{philabs,cucard,pegasus,ihnp4,rocky2}!aecom!naftoli