On later maps, Lan is a small collection of largish houses, so isaf could be the lower house in the collection. I am not a Welsh speaker, so mutations are beyond me, though understand the basics. There must be a transcription issue here, as it will not be Van if written by a Welsh speaker. Lan as a Welsh word, can be translated as; up, bank, side or church. 'Plasbach Van Issa' makes sense apart from the letter V. Is the original document available?