To my knowledge and going to many shows plus being around when the cars were new the solo Dodge emblem that appears hand written was the only one I can remember being on the lid. Never saw any new 70 cars with the Dart Swinger emblems on the trunk lid but there seems to be many around these days.
It all comes down to what you are trying to achieve with your car. Unless you are going absolute 100 point resto it really does not matter in reality. Are you going to put the hood emblem on the front center of the hood too. Many dont have the emblem on the hood which was corect and on 70 Swingers but many leave them off for reasons of their own.
Take a look at a show. Some 70 Dart Swinger 340s have 3 piece trunk and extension trim, 3 piece hood and extension trim, side moulding trim, wheel well trim and bumperettes. Some dont. some have a mishmosh of a little of this and that. You can get confused real fast. The only true answer lies in factory and dealer brochures. These are time correct pics and pretty much gives you the 'skinny' on emblems and trim as everyone has a different opinion on what should or should not be on the car. Just like the grille blackout treatment on the 70 models. Some have it and some dont. I have heard the arguement that only the 340 models had the blackout grille treatment and the low end Custom model had the all silver grilles but look at the show and you will see many 70 340 Swingers with both types. I can go on and on about the differences as I have beat myself up trying to make mine correct. In the end its all about what you want as just about anything goes in regards to emblems and trim.
Truly, a 70 Swinger 340 was a fast little car and loading it down with lots of extras was not the practice. Bumperettes, mouldings, wheel well trim, carpet padding, sound deadener etc were not used so when you see a 'fully loaded' 70 340 Swinger likely it is owners choice to load it up for cosmetics. Sorry for the long winded answer but this is a quest that goes way back for me.
Chas.