I haven't checked myself so I don't know one way or the other whether the material in the Girdraulics is equivalent to AS1866. However, this post isn't about the particular material, but about heat treating in general.
In case anyone is interested in what the above steps are designed to do, first, "solution heat treating" doesn't involve putting a Girdraulic blade in a bubbling cauldron of some solution. It refers to holding the Al at a high enough temperature to allow all of the alloying element(s) responsible for hardening it -- primarily Cu in this case -- to go into solid solution in the Al. If the Girdraulics are made of L40 the Cu content is <4.0%, and if they are RR56 it's 2.1%. What is happening is the same as putting so much salt in a cup of room temperature water that some is still left as a solid at the bottom of the cup after as much salt as possible has dissolved. Because the water has dissolved as much salt as possible at that temperature, it is a "saturated solution" at that temperature. However, heating the Al (or salt water) to a higher temperature and holding it there long enough allows additional Cu (or salt) to go into solution. That is, a saturated solution of a substance at a high temperature contains a larger quantity of the dissolved material than at lower temperature.
Since Cu diffuses fairly slowly in the Al, rapid quenching doesn't give the excess Cu time to precipitate, resulting in a "supersaturated solution" of the Cu once the Al is back at room temperature. However, even at room temperature the Cu atoms very slowly diffuse through the Al. Since the solution is supersaturated, when the Cu atoms bump into other Cu atoms they precipitate as microscopic Cu inclusions. These Cu inclusions pin the movement of dislocations in the Al. A soft metal is one in which dislocations move easily, and a hard metal is one in which they move with difficulty, so L40 and RR56 are "precipitation hardened" alloys.
Although even at room temperature the excess Cu eventually would precipitate and continue to harden the Al, holding it at an elevated temperature increases the diffusion coefficient of the Cu which allows it to precipitate faster. I didn't take the time to look up actual values but diffusion coefficients in solids typically increase exponentially with temperature, which is why the relatively few number of hours at ~175 oC quoted above seems quite reasonable.
Again, heating the Al to ~500 oC allows more Cu to dissolve in it than would be the case at lower temperature. Quenching in water keeps that Cu in a supersaturated solution, at least for long enough to bend it (if it were only to be machined, hardness wouldn't be an issue). Heating to ~175 oC when finished with the bending allows the Cu to have enough mobility that the excess can precipitate into microscopic "pinning centers" that are responsible for the hardness of the L40/RR65/"Duralumin"/"Hiduminium" class of precipitation hardened alloys.
This precipitation hardening would have taken place anyway at room temperature, or if the Al had been slowly cooled from 500 oC, but the higher temperature allows it to happen much faster. Since the amount of Cu that can be dissolved in Al at 20 oC is lower than at 175 oC (which in turn is lower than 500 oC) additional precipitation will take place over the months and years to follow so the hardness will continue to increase, albeit by not very much.
But, to repeat, while the identification of a modern equivalent alloy and heat treatment in the previous post might well be correct, I have not checked this for myself so my own post is only about the general heat treating process.
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