It really doesn't matter as the only difference is that the Whitworth has a 55 degree thread and the standard has a 60 degree thread. The spacing is the same. As thin as the Luger grip is, with only 3 or 4 threads, the thread angle isn't going to matter. Also, after extensive searching on the internet, that was the only place I found that size available, without ordering from England.
You will have to telephone them to ask questions.
From the internet:
"The Whitworth tap is a designation assigned to taps that use the British unit of measurement for its pitch and size. Named after Sir Joseph Whitworth of Stockport, England, the Whitworth type of threading was not initially meant for fasteners such as screws or bolts. It was first used as the measurement of the threading within a rifle barrel that adds spin to a fired bullet. Due to its ideal helix angle, it allowed bullets to be fired further and more accurately. This helix angle can now be seen it the threading of screws and bolts that follow the standard measurements of British taps. Unlike their American counterparts who use a standard 0.013 mm pitch distance it their threading, Whitworth taps generally use a 0.0005 inch pitch distance in their threading. Although these taps are no longer as common as the metric tap since the US standard has been adopted internationally, Whitworth taps are still available in all types of taps. From thread cutting dies and cutting taps to forming taps and custom dies, the Whitworth tap is still very much in use."
"There are three main divisions of British measurements used in threading screws, bolts and nuts. The first is the British Association or BA series of taps. This is the rough equivalent to the Society of Automotive Engineers or SAE standard taps that use the metric system. These are numbered 1 to 14 in sizing. The second is the British Standard Fine taps or the BSF. This is the counterpart of the SAE Unified National Fine taps. These are available in sizes ranging from 3/16 of an inch to 1 inch, with pitches ranging from 32 for the 3/16th inch tap to 10 for the 1 inch tap. The third is the British Standard Whitworth tap or BSW. These are equated with the SAE Unified National Coarse taps of the metric system taps. These are larger taps that range from 1/8 of an inch to 2 ½ inches in size, with the pitch ranging from 40 for the 1/8 inch tap and 4 for the 2 ½ inch tap. These tap sizes are available at Newman Tools Inc of Canada or their US office located in Hartford, CT, USA.
Another Whitworth tap that offers the quality and effectiveness of a large tool making company is the Whitworth tap from Axminster Tool Center. These tap and die tools from Axminster are made from high quality materials and offer long lasting function. They are available in three forms. There are tapered Whitworth taps to start threading off. Another form is the intermediate Whitworth tap that is used to follow up a tapping job. The third form is the bottoming Whitworth tap that cuts threads from the bottom of a hole that has been previously drilled. These are available in various sizes at approximately $3 to $4 a piece."