only 30 strips of 5+1 printed before the roll ran out and was changed to Machin; they come with error of year on receipt 2006. (Thanks JG for the update).
It is surprising that so many of the previous issues were still in post offices. This suggests either that usage of the Post and Go machines for take-away stamps (rather than "post it now" labels) has been very low at these offices, or that they used the default Machin head stock, rather than the birds pictorials. In theory the Birds 2 (issued 24.01.10) should have been used by 19.05.10 when the Birds 3 (Waterbirds) were issued; and they in turn should have been used by the time the 16.09.10 Birds 4 (Seabirds) were issued.
Of course if one issue had not been exhausted by the time the next was issued, old stock should have been removed from the machines so that collectors could access the new. It is more than likely that Birds 4 will still be in some machines when the sheep arrive on 24 February!