41CL: Can someone to explain this index?

[Podalirius]

Hi, I studied many 41CL manuals and don't understand this:
in the instruction to insert the Library#4 in page 4 on write in Alpha register
"804040-8120" where 804040 is the address of page 4 and 120 is the address of LIB#4.
But the '8' in 8120 what is? It is possible to have other value in place of this?
I searched in sy41cl, mem_ref, cl_memory and cl_extreme manuals.
Thank you for your greatly appreciated help!