Hi Bill,
I have an original Burns set left over from the 1960s and Colin Pryce-Jones of The Rapiers borrowed it to obtain the correct gauges etc. He then had some made up for his Double 6, either by Rotosound or Newtone, who would probably oblige again, although this might prove quite expensive. An easier (and cheaper) alternative for the octave-below side would be to use Ernie Ball baritone or six-string bass strings, as this company offers both types specifically suited to their Silhouette Bass. However, these are round-wound, while the Burns versions were tape-wound, which is why the ever-perfectionist Mr. Pryce-Jones naturally chose the more-correct, but costlier custom-made option!