I contacted Keith Wilfort, head of the Iron Maiden Fan Club for many years, in September 2016 to ask questions about this. After a while he answered (in March 2017). Then I asked him some more plus permission to share what he told. I waited a while but got no answer, so now I feel it would not hurt posting my first questions and his answer here.
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Me:
Dear Keith Wilfort, I and some other people are trying to find out when exactly Nicko temporarily replaced Clive Burr after Clive's father died and returned to the UK. At the moment we think it happened somewhere between 5 and 19 September 1982.
If I may ask, what are your recollections? Do you remember when that happened? Unfortunately we do not know recordings from a gig with Nicko during the Beast of the Road tour. Also no photos. Your help is much appreciated. If interested read here and join if you like. Robert Ellis, the photographer, joined today and he conformed Clive was still in the band during the first 3 gigs after Reading. http://forum.maidenfans.com/threads/why-did-steve-harris-fire-clive-burr.32887/ best regards
Keith Wilfort:
Sorry it's taken me a while to reply. But the links you posted were pretty much on the ball. Clive was getting a bit too involved in the rock and roll excesses (much as Paul did). After his dad died and he also split up with his long term girlfriend, he really over indulged. To get Clive back on course would have resulted in a six month lay-off and at that stage in their career it would have been disastrous to shut down for that long, with no guarantee Clive would be back to his best. Therefore Nicko was in the frame. He got on well with the band, had known Rod for years and was an incredible drummer. However, I only remember Nicko siting in for the German TV show and a couple of promo appearances with an Eddie mask. He never actually played a in proper gig on the tour. Nicko was never asked to formally replace Clive until the end of 1982.
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So according to Keith Wilfort, Nicko never played a performance (involving real live drumming) with Maiden, before he officially joined. It makes sense to me, since we (or at least I!) have never heard a recording on which Nicko can be heard. Also no other info pointing out that Nicko had performed, or that Clive was not with the band during a gig. And the death date from Clive's father's death certificate info was already pointing at this, of course, although it did not rule out that Nicko played live with the band. That's why it's good to have the recollections from at least one person very near to the band.