But Handsworth's comment isn't necessarily daft, Borbokis. It is impossible to prove either way at this distance, and as you say you weren't born at the time, but you have to appreciate the particular context in which that match took place In 1979.
Throughout most of the mid to late 70's, Wednesday's support was a standing joke; 7-8,000 gates were common-place. They would get an occasional big crowd which would boost the average a touch, because they had a big capacity to accommodate enormous followings like the 20-odd thousand from Man U when they took the ground over in their Second division title season in 74/75, or the 15,000 plus that Villa brought when Wednesday actually allocated their Kop to Villa and told their own fans to go in the North and South stands to avoid a repeat of the sort of trouble that Man U wrought.
When United went there in 1979, we were riding high and confidently expecting to beat them. We'd also had near enough a decade of being the top dog in the city on the pitch and with bigger gates. Even though we'd joined them in the 3rd division, we were still getting the bigger gates in the run-up - 16,000's compared to 11,000 or so. The Pigs' gate for the home match before Boxing Day was 11,530 against Exeter.
In 1979, the Leppings Lane end capacity was 18,000 and it was rammed. As others have mentioned, there were also United fans all round the other parts of the ground. Obviously, it isn't possible to say how many and as we know, the Pigs have always been able to pull on lots of grannies and aunties for a one-off match.
The Pigs' gates for their next 3 home matches, even after kicking us off the park, were 13,287, 11,958, and 12,425. In the 5 days after Boxing Day, and despite losing to our hated rivals, we pulled in 21,684 against Grimsby and 20,820 against Blackburn. That is a reasonable indicator of the relative levels of support at that time, and that carried on into February until it had become very apparent that Boxing Day had indeed been a watershed in the fortunes of the clubs; that we were in serious decline and the Pigs were on the way up.
The Boxing Day crowd was, in all likelihood, probably not far off being a 50/50 split. One game when United were definitely in the majority at the Sty, however, was Easter Monday in the 1971 promotion season. 47,000 there that day: Pigs were doing nothing and generally down to 13-14,000 and we were heading for promotion. United fans everywhere that day. God knows where the home fans were, but it certainly wasn't on the Kop.