I have both Fast Tech Origin mods.
Stainless Steel - basically like UVO original V1 design.
Brass - which is like the UVO V2 design (missing the extra kick tube - click
here if you don't know what I mean - see the other tube?).
There are a few variances in length of tubes, thickness of switches and lock rings.
The tube lengths vary more than the thickness of the switches and lock rings (which are minimal).
The stainless is slightly taller than the brass model.
In the switch - the stainless model uses an o-ring to seat the delrin battery isolator - which also adds a little height to the battery within the tube. The brass model does not use and o-ring on the delrin - and the threads fit tighter than the one in the stainless steel model.
The stainless steel uses a round head on the main battery contact - this part is interchangeable with the brass model which uses a copper contact. Oddly the lock rings on the switch shafts are threaded differently. The stainless steel model has a silver plated lock ring. The brass has a copper ring.
The stainless steel model uses a spring in the switch where the brass model uses magnets (some complained about their magnets being broken).
I purchased magnets from FatDaddyVapes - because I couldn't find any in that size (I asked someone on FT forums to measure theirs).
The FatDaddy magnets fit well in the stainless model.
I decided to try them on the Brass model just for kicks (see if they "performed" any better).
Well, it turns out that the shaft on the switch of the brass model has a thick base that the stainless steel model does not.
One of the stock magnets on the brass model has a slightly larger inner diameter to slide over that shaft base. The other upper magnet is the same size as the FatDaddy magnets.
FatDaddyVapes magnets: 12mm o.d.; 8.5mm i.d.; 1.5mm thick
Brass Origin: Lower magnet 12mm o.d.; 9mm i.d.; 1.5mm thick (upper magnet is the same as FatDaddyVapes).
The FatDaddy magnets will still work on the Brass Origin.
But the lower magnet will sit on top of that thicker base - which will push the magnets closer together, causing stiffer tension when pressing the switch.
See the difference in height
See the difference in threading on the switch post (for lock ring);
The base thickness (right above the batter vent hole)
Stock brass lower magnet
Brass with either top magnet or FatDaddy Magnet