The standard error are available on the output from x12 (using proc menu from the series):
FORECASTING
Origin 2012.Aug
Number 12
Forecasts and Standard Errors
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Standard
Date Forecast Error
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2012.Sep 175.20 2.708
2012.Oct 175.68 3.242
2012.Nov 176.56 3.700
2012.Dec 177.39 4.107
2013.Jan 178.67 4.477
2013.Feb 178.53 4.819
2013.Mar 179.20 5.138
2013.Apr 180.72 5.439
2013.May 181.47 5.723
2013.Jun 182.93 5.995
2013.Jul 183.70 6.254
2013.Aug 183.70 6.503
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So, its not possible to extract them as a series via command line?