Point of order, and a little pedantic maybe, but our club sources suggest we have had only had 6 sub 10,000 league attendance averages and 5 of those were in the 19th century. So if our league championship and FA Cups are no longer relevant I would suggest neither are 19th century average attendances!
I assume you got your information from EFS Attendances (
http://www.european-football-statistics.co.uk/attn.htm). I believe that many of the very old attendance figures were estimated, and so the figures quoted for the end of season averages are not likely to be accurate.
Anyway, since 1900 we have only had one end of season average (league) attendance below 10,000. That was in 1986-7 season and we were only 8 short of reaching that figure.
If EFS are to be believed, you had 3 post World War 2 sub 10,000 season averages. As with us, these occurred in the 1980s. It doesn't really surprise me that both clubs have had their lowest post war attendances during that decade because football attendances in general were very low at that time.
Incidentally, I see your lowest average was 5,135 in 1984-5. That seems to be a very low average even for that time, especially when taking into account you were still a second tier side. Was there any particular reason for that other than the very high unemployment in the area at that time?