My IBM x31 keyboard has back and forward buttons above the left and right
arrows, there should be entries for these so that they map to XF86Back and
XF86Forward, I suggest the following changes:

xkb/symbols/inet, add:

partial alphanumeric_keys
xkb_symbols "thinkpad" {
    key    {       [       XF86Back                ]       };
    key    {       [       XF86Forward             ]       };
};

and add 'thinkpad' to the list of $inetkbds in the rules/base file.