HI Ken,
Maybe this will answer that query! to quote from the Flight article:
When Heino Brditschka flew the MB-E1 for 9min 5sec from Linz in Austria on October 21 he qualified for a line in the record book. For, unlike the HB-3 from which it was converted, the MB-E1 (Militky Brditschka Electric One) was silent.
I read that as saying the HB-3 originally flew under I/C power and it was then converted to battery electric power and was renamed the MB-E1, still carrying the same Registration number.