The introduction of the new bronze halfpenny in Great Britain in 1860 saw many different obverse and reverse die iterations produced and used in an attempt to get a satisfactory die life. One of the major design changes was to replace the beads around the rim of the obverse and reverse with denticles/teeth that ran into the rim. Clearly there was not a distinct transition however as there is one known 1860-dated halfpenny that was struck with Freeman obverse 2 (denticles/teeth) and Freeman reverse A (beads) which is classified as Freeman 260C. The coin in question shows a die crack through the top bar of the T in VICTORIA that runs parallel to the vertical stroke.
The single known toothed/beaded border mule was a part of the Laurie Bamford Collection and was also sold by Dix Noonan Webb as lot 208 in its February 12th 2020 auction. There has only been a single known example for many years but the existence of another example cannot be discounted.
The first obverse die used to strike Queen Elizabeth II New Zealand pennies was found to be unsatisfactory as the portrait was of too high relief causing the shoulder strap on the Queen's clothing to appear indistinct or missing as a result (p72, Pepping, New Zealand History Coined, 2017). A second obverse die with a clearer shoulder strap was prepared and used on the coinage of Great Britain from 1954 onwards but was not deployed for the striking of New Zealand coins until 1956. The 1956 penny was struck using both the old 'no shoulder strap' and new 'shoulder strap' obverse dies (p73, Pepping, New Zealand History Coined, 2017). The 'no shoulder strap' pennies can be identified by the second stroke of the U in QUEEN pointing between rim denticles while the shoulder strap pennies can be identified by the second stroke of the U in QUEEN pointing at a rim denticle.
A total of 3,600,000 1956-dated pennies were struck but it is not known how many were struck using the old 'no shoulder strap' dies. It is estimated that just 50,000 coins were produced using the old design (p7, Mitchell, The New Zealand Coin & Banknote Catalogue, 2007) making it a scarce variety.
The reverse of the Australian 5c has undergone a number of changes since its introduction in 1966. From 1991 until 1994 a new reverse die was deployed but unlike other changes to the 5c reverse, the change was ultimately not adopted. The new reverse's only difference appears to be the enlargement of the designer's initials SD under the echidna, with the letters SD appearing large and more elongated.
Small and Large SD 5c circulation break-downs
The relative rarities vary by year but the figures suggest that the new reverse was trialed in 1991 and 1992 and that in an effort to not let old dies go to waste, the supply of remaining large SD reverse dies was used through 1993 and 1994. The large SD reverse has not been seen in any other years.
Since the advent of the Ian Rank-Broadley obverse in 1999, the obverse used on Australia's decimal coins has changed subtly but frequently. Usually the changes have occurred as the years have changed, though in a few cases the changes have occurred during the production of a year's coinage. In 2007 for example, the obverse design was changed part-way through the production of 20c pieces.
2007 20c circulation break-downs
Round 2/thick 7
Blunt 2/thin 7
On the first design for the year the date has a rounded tip on the 2 and a thick vertical stroke on the 7 while on the second design for the year the date has a flat tip on the 2 and a thin vertical stroke on the 7. Both varieties are common in circulation though the second variety is slightly more plentiful.
In Dix Noonan Webb's March 2011 auction, an 1837 sixpence muled with a William IV half sovereign obverse was sold at lot 180 for £680 against a £200 - £300 estimate. So far it is the only known example of an 1837 mule sixpence and while the listing said it was not listed in any of the main references it is now listed as ESC 2514 in English Silver Coins.
The circumstances in which it was struck are not known but it was likely a die mix-up. The Gold Half Sovereign by Michael A. Marsh lists MAM 412A as an 1836 half sovereign struck with a sixpence obverse die, and while the circumstances in which it were struck are also unknown, Marsh speculates that the sixpence obverse die "was available so therefore why not use it" (p16, Marsh, The Gold Half Sovereign, 2004). While this is possible, given the similarity between the sixpence and half sovereign obverses, it could be that there was a die mix-up in 1836 with a sixpence obverse die being used to strike some 1836 half sovereigns and a half sovereign obverse die being used to strike some 1837 sixpences.
While the Alfred Bole Collection example that was sold in Dix Noonan Webb's March 2011 auction is the only example known, there are almost certainly other examples in existence. It is unclear how many coins a standard sixpence die could produce in the 1830s but it seems probable that the mule is the result of an entire die run yielding probably tens of thousands of mules.