Revision 62b30d82-34d9-434e-8fb1-868c8da1b979 - Code Golf Stack Exchange

PHP, 198 bytes
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 <?if(substr($argv[1],-2,1)=="\n")$argv[1]=strrev($argv[1]);foreach(explode("\n",$argv[1]) as$l){$s=strlen($l);$x[$s]=++$i;$m=$i==$s?'T':'M';}$z=count($x);echo $z==1&&key($x)==$i?'S':($z==1?'R':$m)?>


The upside down triangle adds an expensive 57 bytes to this!

<a href="https://tio.run/##PcvRbsIgFMbxe56iIScCsZp0l8WzPoE3ujtnmq6ikFUggKbbslcfa5fMy/M/v89rn/OmMWceb28xBQ5duNwP1bFcPZWVQKSvlor/iJMI6v5AQp5dUF2vuRr94E6Kz7x8vIsuwiC@IM7DQVk@XRLGA8QjLpdgJFwRDCLEhr2wmm2Z/IZP7N3NJg6jkKrXrpgKVovFu/qY26RNw/as5n@9YTtWw1U0zzlnIC0SunckaRMJXVO6JlF3Xskf55NxNubVqYjahdQ6r2ybugs6@ws" title="PHP – Try It Online">Try it online!</a>