Wikipedia:Revision ID
Every historical version of a page has a unique revision ID, which you can find from the history of the page. Click the date and time link of any entry in the Revision History list, and when the page for that entry is displayed, the URL at the top of the screen will show the revision ID at the end of the line after the “=” sign.
For instance: https://en.wikipedia.org/w/index.php?title=George_Formby&oldid=624145252
Given a page name and one or two revision IDs, you can construct links to several types of diffs. Using Template:Diff is one way of achieving this.
If you just want to link to an older version of a page without reference to a diff, you can use Template:oldid2, which also will allow you to link to a specific section in an old version. For instance:
{{oldid2|624145252}}
produces- [1]
{{oldid2|624145252|George Formby (as revised 11:20, 4 September 2014)}}
produces- George Formby (as revised 11:20, 4 September 2014)
{{oldid2|624145252|Screen persona and technique|}}
produces- [2]
{{oldid2|624145252|Screen persona and technique|Formby's stage style (article version 2014-09-02T11:20)}}
produces- Formby's stage style (text as of 2014-09-02T11:20)
#1 and #2 both link to the same article as the complete URL in the second paragraph; #3 and #4 link to the section titled "Screen persona and technique".