A Month1 / Week1 tax code usually suggests that the person has had some other form of income during that tax year. This could be a second job, leaving one job midway through the year to start another, pension income or the person has been unemployed. If your P60 shows this tax code and you did receive another income, we will require additional proof of the income received during the relevant tax year.
If a 'Basic Rate' BR tax code is shown it usually indicates there is a second form of income from employment (which would have a 'normal' tax code applied). Therefore additional evidence (P.60 / month 12 / week 52 payslip / P45) will be required or a letter (statement of earnings) from HMRC detailing the total taxable income and tax paid.
HMRC can be contacted by calling 0300 2003300. Also if a 'Basic Rate' OT tax code is shown it usually indicates that all the tax allowance has been used up and usually indicates there is some other form of taxable income.
Although some P60s are different, a Month1 / Week1 tax code will normally be indicated by M1, W1, X or 1 printed after the tax code.
You can find more information on tax codes on the gov.uk website.