(the "timely mailed/timely filed" rule inapplicable to amended returns that show increase in tax due; I.R.C. Sec. 7402 only applicable to returns "required to be filed"; submissions of amended returns that are timely when mailed, but beyond statute of limitations for assessment of tax when received, cannot be assessed tax for increased tax shown on return; "timely filed/timely mailed" rule does apply to amended returns which include claim for refund).