Plastic hull shotshells first appeared in the 50s. Remington SP-12 is a line of Rem shotshells from no earlier than the 80s, or possibly 90s, I forget, but I have a box.
The SP line came after the "Shur-Shot" and "Express" shells and if I remember correctly one of the things offered in SP12 shells was duplex (dual size) shot loads. The box I have is marked BB X4 and I think its BBs and #4 shot mix.
3 3/4 dram equivalent is the old "Express" load and the smokeless match to the speed and pressure of 3 3/4 drams of black powder. That was the top level power load for 12ga until the creation of the "baby magnum" 2 3/4" shells.
The Shur-Shot line was lower power, being 3 or 3 1/4 dr eq. loads.
As to birdshot for home defense, at across the room ranges, the pattern isn't going to be bigger than your hand, forget the old BS that "you can't miss" or "you don't need to aim". You CAN miss and you DO need to aim.
Many will say you need buckshot or bigger, and there's no question it does work. But considering the point of defensive shooting is to STOP the attack, not to kill the attacker, birdshot can do that, if you hit them in the right place. If they die as a result of being stopped, that's a tragic result of their bad judgement.
If you can get buck, by all means, use it in gravest extreme. But if all you have in that situation is birdshot, don't think you're not armed with a deadly weapon, you are.
