DRupp

Option Explicit

Private Sub Worksheet_Change(ByVal Target As Range)
Dim serial As Variant 'current serial number

'turn off events to stop inadvertent looping
Application.EnableEvents = False

' assumes that serial number will be created when column B is altered
If Not Intersect(Target, Range("B:B")) Is Nothing Then

    'extract the serial number
    serial = Replace(ThisWorkbook.Names("SerialNumber").Value, "=", "") + 0

    'apply to column C (adjust to suit)
    Range("C" & Target.Row).Value = "M-" & serial & Format(Date, "ddmmyy")

    'increment serial number
    ThisWorkbook.Names("SerialNumber").RefersTo = serial + 1
End If

'restore events
Application.EnableEvents = True
End Sub