Oh, MR is most definitely Peninsular War at the very earliest. Terry has been moving Discworld along so that now, after Going Postal, it's in the height of the late Victorian period - and Thud implies that at least child-rearing has moved into the early to mid 20th century. Although Sybil was *always* there. You could stick Sybil in a Sayers book and no-one would blink twice. She could be Peter Wimsey's aunt quite easily.