NAL, but this seems pretty clear to me
SECTION 157. The department of criminal justice information services shall establish
2445 the electronic firearms registration system established pursuant to section 121B of chapter 140 of
2446 the General Laws, as inserted by section 32, not later than 1 year after the effective date of this
2447 act; provided, that all firearms shall be registered in accordance with this act and not later than 1
2448 year after said electronic firearms registration system is completed and publicly available
I read that basically as: Within 1 year of the law going into effect the registration system must be created and within 1 year of the system being created, all firearms transfers have to meet the conditions required under 121B. It appears retroactive because of the wording "Not later than 1 year after said electronic firearms registration system is completed". The law is very clear that it's 7 days. The only reason to put that sentence in, is to provide for retroactive registration.
Specifically:
409 All firearm transactions shall be reported by all parties to the
410 transaction via the electronic firearms registration system within 7 days of the sale, rental, lease,
411 loan or other transfer.