The Bivins T.G. is a contemporary creation and doesn't , in my opinion , resemble a Dickert. Jack Brooks sells an exact copy of a Dickert T.G. circa 1780. The original had a single trigger but there's plenty of room for a set.
Also Drew Hedgecock sells a T.G. of a Dickert from the 1770's. TOW sells this as TG-K-1-B. Very large. On my website, 1780 Dickert uses this heavily modified.
Reeves Goering has no T.G.'s by Dickert for sets. However, he has one by John Brooks that with some modification, would be a good Dickert 1790's -1800. No. 43.