Certainly this means a lavatory in the original sense of a vessel used for washing. It's a little confusing in that the plans also use the same word in its later sense of a place for washing and/or performing one's bodily functions. The use of terms like 'lavatory' and 'toilet' (which originally pertained to dressing and grooming) as euphemisms for other activities 'not referenced in polite conversation' were generally confusing at that time when the more traditional meanings were still in common use. Even today in the UK we are confused by American requests to use the 'bathroom'. "You want to take a bath? Now? Oh, you wanted a ****? Why didn't you say so?"