[FFmpeg-user] Some Videos Display Narrowed (Squared) Image

[Budge]

I have a set of Videos, most of which are quite old and in 4x3 format.
Even so, when playing on wide screen some videos are displayed correctly 
with correct proportions and some are displayed with a narrower image so 
that picture is virtually square and distorted slightly.

Checking with mediainfo on master copy I can only see minor changes, in 
particular:-

Writing application Lav57.41.100 vs narrow version Lav56.40.101
A bit rate of 1384 kb/s vs narrower 1479 kb/s
threads=13 / lookahead_threads=2 vs narrow threads=8 / lookahead_threads=1

Please could you identify problem and possible solution.
Many thanks,
Budge