ART & ENTERTAINMENT: I've a British friend who writes me: There's a
film I've been trying desperately to track down.  It had no
voice-over, no dialogue, no music...and was maybe 30-40 minutes long
(certainly shorter than feature length) and it played in the UK as a
second-feature back in the late 60's, I believe, or early 70's. It was
a dramatised documentary in which an Native American sets eyes on a
horse for the first time, then gradually built up a
Monty-Roberts-style relationship with it, ending with him riding it.
It had a "the rest is history" type ending as I recall. It was
absolutely magical.  No one on IMDb or at the British Film Institute
archive was able to identify it.

QUESTION: Does this ring a bell with anyone?  Title or director?

Hello.

I believe that the film was "Conquista" (1974).

This 20-minute color film was released in 1974.

See this description:

"Conquista [motion picture] / Alan Landsburg Productions 
[1974]
...Description : film reel (20 min.) : sd., col. ; 16 mm ...
...This film depicts what might have happened when an American Indian
brave of the 16th century encountered the first horse he has ever seen
and discovered how to ride."
source: State Library of Florida Catalog
http://draweb.dos.state.fl.us/web2/tramp2.exe/log_in?SETTING_KEY=English
[enter "conquista" in the search box and press the "title" button. The
film will be the first entry in the list of results]

search strategy:
library catalogs: horses, indians

I hope this helps.