That's a very interesting lock and an interesting key profile as well- especially considering the age of it- it's definitely got a Victorian Bi lock look about it.

On the subject of making blanks- Dean's right in that it's definitely going to involve knowledge of the lock and working with metal.

My first choice would definitely be to profile the section from solid brass rectangular bar- once it's filed or milled to size (might even be lucky and find some already the right size). Milling down the centre will then create the 'U' shape you need. The photo seems a bit unclear to me though- it looks like the bottom of the 'U' might have a shaped profile on it? If so it's presumably to stop a basic 'U' shape blank from fitting. This profile can again be milled or carefully filed by hand depending on how adventurous/skilled you are. An advantage of this method is that the other end of the bar can then be milled down, the end turned to form a round spigot, and any key bow you have or want to make, can then be nicely brazed on the end. Or, the bow end can be left slightly wider or 'flared' where it projects from the plug, and before you turn the spigot. This will act as a collar or 'stop' for the key sliding in the plug.

Obviously all the profile and shaping has to be done step-at-a time and very accurately to fit the profile and the plug though, as any wobble or slop will render the blank useless as a working key- And, on an old lock cylinder of that age it's anyone's guess as to how worn the internal profile could be. You simply won't know until to do it!

Dean's method is another good one using a 'built-up' process to create the shape, and yet another I'd consider would be to fold the 'U' section up from brass or nickel silver strip of the correct thickness. Easy enough to do quite tight cold bends in brass, but it's easily annealed to soften it to make things easier and get the corners even tighter. With both these methods you will have a bit of faffing around to fit the bow on though, as Dean said.

Also don't forget that after all that's done it's still a big unknown as to the condition of the lock inside- unless you've already had it apart of course. That would make things a lot easier for making the blanks and sizing them to the plug.

It's the sort of interesting and relaxing job that most of us would probably keep on the bench for ages and just pick it up every now and then and do a bit more. these sort of 'unknown' jobs are fine when you can do it all yourself but could be problematic and expensive if someone else has to be paid to do it- there's also the potential for a lot of fiddly little worn out parts and springs inside, in addition to hand making the blanks and making the keys, so fingers crossed that none of it is worn and that all the parts inside are ok. Give it a go and let us know how you get on