Can you explain how the Massachusetts Retirement Reduction is calculated from a 401K contribution reported on a W2, when there is no 1099R?

[linear]

401k contributions made pre-tax do not count as taxable income in MA (same as federal treatment). MA allows a deduction (up to $2000) for amounts contributed to Social Security and Medicare - there is no other deduction for 401K contributions.

[DaDa]

Thank you.  That explains it!