Also, under the old Fed Ban (all the info we have to work with), the pre-ban lower to actually be recognized as "pre-ban" HAD to either have been built up or kitted with "evil features" on/prior to 9/13/1994 to count as "pre-ban" legally. No such ruling has ever occurred TTBOMK in MA, but that precedent could easily be used in court and I'd give odds that it would be accepted if ever put to the test.
As pointed out above if you live in Boston, you can't possess ANY AR-15 inside the City limits (or larger than 10 rd rifle mags for anything)! Boston doesn't recognize anything as "pre-ban". That said, if you can store it outside of the City and never bring it into the City, you'd be legal to own it.