I am using A Concise Dictionary of Old Icelandic and An Introduction to Old Norse (by E. V. Gordon) as my resoources.
In An Introduction, it is said that:
Some nouns declined otherwise as masculine a-stems had the gen. sg. in -ar, or -s interchanging with -ar, as skógr, smiðr (nom. pl. -ir and -ar), vegr...
I don't understand the text in bold at all. Does it mean the gen. sg. ending might be either -s or -ar, or does it mean something else?
Also, in A Dictionary, I find (-s, -ar) written right next to noun entries, like this:
stigr or stígr (-s, ar; pl. -ar, -ir, acc. - a, -u), m. path.
staurr (-s, -ar), m. pale, stake.
diskr (-s, -ar), m. plate, dish.
Is this related to the gen. sg. ending?