Looking a little closer and also in older index files i found this.
*****A "C" Following a Project Gutenberg eBook Number Indicates Copyri
+ght****
*****A "*" Following a Project Gutenberg eBook Number Indicates Reserv
+ed ****
[snip]
The Life of John Ruskin, by W. G. Collingwood
+ 13076
A Hero and a Great Man, by Francis Kruckvich
+ 13075C
[Illustrator: Fritz]
Punch, Vol. 100, February 7, 1891, Ed. by Sir Francis Burnand
+ 13074
[snip]
Feb 1995 Moon and Sixpence by Somerset Maugham [Maugham #1][moonaxxx.x
+xx] 222
Feb 1995 The Return of Sherlock Holmes [Magazine Edition] [rholmxxb.x
+xx] 221B
Feb 1995 The Secret Sharer, by Joseph Conrad [Conrad #2] [ssharxxx.x
+xx] 220
I did not found the meaning of the 'B' though.