I've just been looking at a download from the Mercian Register and it looks like from 1958 to the mid 60's the serials went date prefix. Judging by the few examples on the register.
See extract below
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#59574
1959 Campionissimo
Blue (polychromatic?), lugs lined in gold 20-1/2 c-t
#59709
1959
Nigel Olson, UK ’07 June 2006
1961
#61685 unknown model, orange
sold by Mel Blake of Southport, England in May, 2004
#6191
1961 road-path
Nigel Olson, UK
07 June 2006
#61274
1961 Unconfirmed as Mercian pics from eBay
#61831
1961 Olympique [confirmed] Ian Skinner, U.K.
Serial Number 61831 - confirmed previously by Mercian as a 1961 model
1962
#48262
David Buxton, UK
07 June ‘06
Possibly a Campionissimo, based on info from #63125 **See below for explanation of raised digit in serial number
Hello,
I bought a Mercian from the local Tip for £2. It was in a Skip waiting top be crushed! It has a Stronglight drilled chainset and
rides beautifully. The number is. 48262 and a raised 2 at the end. The following is part of an Email from Mercian at Derby. "Blimey! Don't quite know what to say about a Mercian being tipped.
Anyway - the 482 represents the number - i.e. 482nd frame - built in 1962. The number 2 slightly higher up represents the frame builder - Cyril Wagstaff, who retired from here about 5 years ago. We don't still have the records going back quite that far, so unfortunately I can't tell you who owned it originally, but it would certainly be Reynolds 531 tubing."
I have attached a picture to show how it now looks with a little cleaning up. Hope this helps,
All the best,
David Buxton
1963
#63125
1963 road-path [Campionissimo?] Nigel Olson, UK
’07 June 2006
my Mercian Road/Path bike Serial number 63125, renovated by Mercian this year, they think it is a Campionissimo?
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I thought I'd leave in the comments about the one found in the tip.