According to the Instructions for Maryland Form 502, this subtraction from income is available for "Payments from a pension system to firemen and policemen for job related injuries or disabilities (but not more than the amount of such payments included in your total income)." See page 15, under Code Letter a.
Nowhere does it specify that this pension must be sourced in Maryland. You would be entitled to this subtraction if you are receiving such a disability pension, no matter which state it is from.
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