If you look in the SNDB it says HBG 0000000 confirmed by BOC.
At the beginning of each Introduction to a new series, Charlton introduces information that it has acquired on that particular series. It typically has a chapter under "PREFIXES AND NUMBERING"
and:
"Each series consists of numbers from 0000000 to 9999999, the zero being removed and a replacement note substituted." P349
-Perhaps the SNDB should have a similar qualifier to their charts (but leave out the
replacement note substituted qualifier for commemorative & vertical series).
Also for their CHECKLIST OF PREFIX LETTERS FOR POLYMER SERIES, (P350-351)
an * is placed beside Serial Numbers where observations differ from BOC statements.
At the end of the chart:
* Denotes high and low serial numbers seen
I have made the suggestion that GPM32 begin to qualify the discrepancies found in the M/C prefixes HBG; FTH, BSW, AMK & GHD.