I was privileged in a college Sweeney production to work with original Bway set designer Eugene Lee, who provided us with the original then-20 year old razors from the 1979 Broadway production. The handles were about 7 inches long, made of wood with an oval opening on the underside. The opening was about five inches long - enough to get all of your fingers inside to push down on the squeezy bladder inside.
The bladder was cylindrical and reusable, the same sort of material used in the bulbs of blood pressure cuffs. They were filled by syringes with Rosco blood diluted with water - not sure of the dilution we used as it was over a decade ago now, but I do remember trying several different densities. Too thin and you saw a stream of blood fly out towards the rail if Sweeney was feeling particularly energetic! Too thick and you couldn't get it out of the Judge's white ruffled collar.
The blade mechanism screwed on once the bladder was filled. It was a simple hollow metal tube about 3/8" diameter with a threaded end, then a wider metal flange, then about a 15 degree bend to make it look realistic and also help keep the blood from sloshing all over until it was needed. The tube served as the spine of the razor. It was punctured with small squirt holes about an inch apart all the way down the length.
The tube was then wrapped both sides in sheet metal (I'm guessing it was aluminum, possibly steel) bent into a U shape like a taco and secured onto the unpunctured side of the tube. The metal stuck out about an inch on both sides from the tube, folded parallel but not quite touching so that from the house it looked just like a very thick razor blade. The sheet metal was blunted, of course. It served as a pane to help smear the blood coming out of the tiny holes and make it look more uniform.
We used three razors - two from the original show, and then we built a one for the contest which stayed empty so that there was no chance of it ever "bleeding" during that scene if someone forgot to dry it off after washing. Having the smaller one made it easier for our Sweeney to make the dainty motions necessary for the contest music. He would also use the empty one if he had to do any stropping or heavy motion with a razor - swapping out razors can certainly be made into believable business. The other two were filled, as we didn't have time in our particular staging to refill them between the Beadle's and Judge's deaths.
With the tiny pinholes in the tubing it was definitely crucial to wash the things thoroughly after every performance, or they'd clog with the fake blood. The sheet metal "blades" were quite fragile, as I recall, as was the threading that secured the blade to the handle. Given that you're dealing with goop inside, you definitely do not want to strip that particular thread!
Hope this helps.