In Deirdre Le Fay's Jane Austen: the world of her novels, it says that the Derbyshire militia were in Hertfordshire at the time JA was writing P&P. They may be the basis of the unnamed militia in Meryton. "As the militia were recruited on a county basis, Darcy would probably know most of the officers of the regiment and so could suggest to Bingley that the two of them should follow their friends for society's sake, and look for houses at the same time" (189).