I have yet to find one in the wild but I suspect there were quite a few of these brushes made. I do look every time I'm in an antique shop.
The one you found is in really nice shape. Depending on how far you want to take it will determine the amount of effort you'll want to put into it. I'd start out by using some Mother's aluminum polish as it'll give it that mirror like shine. Any imperfections will show up as dark spots but you'll likely have to look close to find them. Once you shine it up you'll know where you're at as far as any imperfections go. I bet it'll make a very large difference. I went for perfect on all of my 400's and they look outstanding. I suspect you could get this one to perfection quite easily. Do find the Rubberset 400 thread for all the information you'd need to refinish this knot including how to easily get it apart and remove the old knot as well as what to use in terms of sanding and where to get the rubber o-rings.
As to the knot it depends on your budget and what you like in a knot. I have four of these brushes so I'll tell you what knots I put in them.
The knots that are in mine in order of preference are:
Declaration Brushworks (L&L Grooming) - A really outstanding knot. This one as well as the Elite knot are in my opinion on par with the Shavemac and other really high end knots. This knot ran me around the $100 mark so it's on par with the Shavemac cost wise as well.
Elite Manchurian White Fan - This one is outstanding and a very good value for what you get. I did have to sand the opening a little to get this one to fit. This one was about $60 but it holds it own against the higher priced knots I've bought. I reach for this one a lot as well as the Declaration knot.
TGN High Mountain White - Another really nice knot very soft and luxurious. I was a big fan of this one but I'm liking a little more backbone now. It does shed quite a bit at first because it's very densely packed with thin hair. This one was somewhere around the $55 to $60 range (I think)
TGN Finest knot - this one is ok but not quite as nice as the others. Then again I think it was in the $25 to $30 range when I bought it.
Other knots to consider...
The Envy Luxury knot is one I've bought and installed in another brush handle but not in a 400. I like this knot quite a bit as well. It's another excellent value knot at around $36.
I've also put a Shavemac 2 band Silvertip (not the d01) in another non 400 handle. I might put one of these in the fan shape in a future 400 if I happen to stumble across one. Many consider this knot to be one of the very best available as it's frequently recommended when people ask for a high quality knot. It would be in the $115 range for this knot.
I concur. Love it when things stay period correct.Best of luck- I hope it breaks in nicely and I somewhat selfishly pray that you get many years of enjoyment from it as a boar brush. While I understand how someone would choose to put a badger knot into these vintage brush handles the history aficionado in me always feels a little sad whenever I see that the original knot wasn't replaced by another big boar.