TableCaptionAlign Enum missing in Same Version of two System.Web.D

I have 2 System.Web.dll taken from 2 different machines, one of the
System.Web.Dll contains a enum System.Web.UI.WebControls.TableCaptionAlign
and the other did not. I have development a functionality which was not
working in some of the machines and on diggin' into, i found this. I also
compare the 2 dlls through ILDisassembler. Can anyone explain me WHY, WHICH
one should i use?