Good question Al!
Here SG's numbering scheme is actually "SG LSxxxx" rather than "SG Lxxxx".
So the two sheets here are SG LS8 (not L58) and SG LS39.
SG calls these "Post Office Label Sheets" and they are generally known as "Smilers" as the first sheet (SG LS1) was made up of the 1981 "Smiles" greetings stamps designs.
The concept was to have a sheet with a number of stamps each with a customisable se-tenant label and these were produced at stamp exhibitions etc.
SG doesn't list the customised/personalised/corporate versions of these sheets, just the default "generic" format in each case and these are often called "Generic Smilers".
These Post Office Label Sheets have their own section towards the end of the SG Great Britain Concise catalogue.
The stamp in SG LS39 has a changed design from SG 2291 etc. but (at least in my old 2010 catalogue) it hasn't been given a full listing on the grounds that these sheets were sold at a premium over face value.