Paul, the VM says that there were a number of slightly different Process and Apo Process Lustrars, all dialytes. In historical order, f/10 Process Lustrar, f/10 Apo Process Lustrar, and f/10 Apo Process Lustrar Series II. It says that all are sharp but the the first two (plain Process Lustrar, Apo Process Lustrar with no series indication) have relatively low contrast. The Series II is supposed to be by far the best of the lot.
Here's what the VM says about it: "Apo Process Lustrar Series 11 f10 This type was seen in 4.0in, 12in, 18in and 25in. These are still a dialyt type but offer a higher performance and the coating is harder, and are much to be preferred for use. The dialyt can be used from infinity to close up where it was intended to be used. They are seldom in shutters, but are marked Series 11. The 12in has been seen several times. The B.J.A. 1953 p212 noted a 25in example as entirely freshly computed and 'superlatively good' and well up to the 18 and 36in examples of the series. It was really good even for 3 color work at f16- much wider open than usually used in this work. On a 15x12in plate, the biggest colour error was in green, about 0.001in over a 15in dimension. There was no appreciable field curvature and no curvilinear distortion was noted. [It is suggested that this was one of the lenses C.G.Wynne redesigned]."
I have a 16"/10 Apo Process Lustrar Ser. II. It should be coated, but the coating is very hard to discern. I've tried it out hung far, far in front of a Nikon. The news is that its flary and not really sharp until f/22.
If I were you I wouldn't buy the 18 incher you've been offered. Cloudy glass doesn't always come clean. If you're up for a Sinar shutter, I think you should consider looking for an Apo Ronar or dialyte type Apo Nikkor around that focal length. Newer coated lenses, IMO better too. Otherwise a proper taking lens in shutter.
An 18" lens needs 18" of extension to focus to infinity; hanging the lens in front of the board reduced the extension needed by roughly half of the barrel's length. Focusing closer requires more extension.
Good luck, have fun,
Dan