So my question is: which are the securities I acquired first? Are they the original 100 shares I bought years before starting the DRP, or are they the first lots acquired under the DRP?
You can't reinvest dividends on shares you don't have. The answer is obvious.
Regarding my first comment.
As you buy fractional share with dividends the fractional shares combine into one share. If you have a fractional share it must be the last share or shares you bought and if you continued to reinvest until sale that fractional share would get ST tax treatment. As would any other full shares acquired less than one year prior to sale.
The basis of your fractional share when you transferred the securities is the price per share on the last day of reinvesting at the old broker times your fraction.
If you were getting less than one share per dividend period, then the calculation is more complicated.
