We're on a mission from Burr...
Know Ronald Burr's death date would solve definitely the mistery of when Clive was replaced. But i'm afraid we'll have to make the long ride.
Having said that, i believe that a band must take 3 or 4 days off for preparations before do the replacement, or they'll cancel the gig. Also, i believe Nicko wouldn't play a full gig nor would execute any drum solo in the set.
I did my research in 4 steps:
1) 82 audio bootlegs - from May to August (1st North American leg), and from September to October (2nd North American leg)
Definitely the most difficult and erratic task. You'll be exhausted trying to hear something "different". Even if you're a drummer or have trained ears you'll end up bumping into bad quality, slower, dusty recordings, and sometimes you'll hear Clive missing tempos and hitting bad notes, which can lead you into believing it's Nicko. I had to hear some anyway, but can't expect only differences in style.
2) 82 videos
There are only 2 of significance apart of Hammersmith official vid, but they cover an important area.
INSTORE SIGNING - Kalamazoo, U.S.A., May 1982
Kalamazoo instore was filmed only a few days after Maiden enter North America for the 1st time in this tour. Nicko was Trust's drummer, whose band supported Maiden on April in Europe. Then Maiden flew to the USA in May while Nicko stayed in Europe with Trust until his departure around June.
In USA/Canada, Maiden had literally no gap between dates. So instead of rehearsing with Nicko, considering their hyper tight schedule from May to August, they would probably cancel the gig if Burr couldn't make it.
AUSTRALIAN TV. Nov 1982
That's after the 2nd and last North American leg. Clive Burr was in the band after their last visit to the USA as RueMorgue said. As far as i know there are no Australian (real) bootlegs available (the only one from 16-11-1982 is said to be an Oxford gig). So let's say from now that "Burr is back" in this TV show in Australia.
3) 82 pictures
As been said earlier in this topic, Nicko replaced Burr in the summer. I've heard about that too, although i don't recall the source. So I particularly looked after Maiden pics from North America, 1st leg. That should cover the summer part of the tour. We have Burr in June, July and August playing with Maiden. And... he is happy! I don't think he would be happy after his father's death and knowing he would soon leave the band. And interestingly enough, there is this picture here from 21st September in Chicago, where he seems deadly serious. Found out that I don't have a lot of pics with Burr from that part and in all of them he seems fatter and serious. Coincidence or just my imagination?
4) Maiden official biographies
I'm almost sure the replacement thing came into agreement at the Reading Festival. Whether it has now something to do with Burr's father, i don't care, as we don't know the funeral date and I assume we can surpass this subject. Also I'm not claiming it's Reading because of any resemblance with the Paul-Bruce thing. But...who knows? Anyway, I came to a conclusion after a view on Running Free 1985 edition book.
Garry Bushell accompanied the band in their 1st USA leg and covered this performance of the band in Texas on August (page 105). On the same page there is a picture of the author and Burr in Texas, sitting down and smiling. Again, would he be happy if anything like that happened? But then comes what i consider the keypoint: Gary says "H and Murray swapped sizzling guitar pyrotechnics. Dave excelling himself with that smashing first break on 'Number' while Clive Burr contended himself bashing out a solid beat from the rear.
Little did i know that this would be the last time I'd see Clive play live...".
Well, he didn't say it was the last time he saw the BAND play but CLIVE. So here is something! He covered what was Maiden's last gig in the U.S.A. before flying back for a few warm up gigs and the Reading Festival. And yes, Clive did play at the festival, but Gary Bushell apparently wasn't there. On page 108 it shows he was aware of what happened there but he used quotes from Kerrang magazine to describe what happened.
On the next paragraph Gary don't go into details about the 2nd North American leg gigs (why?), only says that Maiden sold out shows in New York and Hammersmith Swan. He didn't go to Australia and Japan, as well. And that's why he saw Clive last playing in Texas. I presume Gary only saw the band in the first European part and in U.S.A.
On a final note, I suggest you to listen the
September 4th, 1982 Oakland bootleg. Only 6 songs and some different drum hitting style in my opinion.