Indeed it would be nice to know!
The most similar I could find is Capital (http://new.myfonts.com/fonts/aboutype/capital/), but it's flared, or Radiant (http://new.myfonts.com/search/name%3Aradiant/fonts/), but it's too round.
In the meantime, hoping someone could find it, here's the recreation of one amateur:
http://theavengers.tv/forever/misc-6.htm

Thanks for the response! Perhaps it's a variant of Universe?

Do you mean Univers (http://new.myfonts.com/fonts/adobe/univers/)?
Rather handmade.

There were some film faces back in the '60s/'70s that were similar--California Grotesk, Ibsen--but I can't find anything with such high contrast between thicks and thins. It's either custom or something really obscure.

The case of font used for The Avengers is interesting, and certainly during Series 4 and Series 5 of the Diana Rigg episodes, the graphic design was of a very high calibre. For the monochrome (Series 4) programmes, I have only ever found the font as mentioned above by riccardO. For Series 5, however, it appears the font used was Compacta, and for the opening fanfare 'The Avengers In Color' it appears to be Compacta Outline. Apart from the title, Compacta was also used in Series 4, along with a combination of Helvetica.