My guess would be that it's the bed from and early to mid 19th century plain turning metal lathe ie no main lead screw or rack, the saddle positioned where needed for working with the cross slide and any longitudinal movement being by using the top side. I think the "flat & inverted v" guide was favoured by the Americans – looks very similar to that used on the Ames Chicopee (as on lathes.co.uk web site). Probably nothing really remarkable I suspect there were many made for use in local workshops but quite a nice piece of old machinery.
The bars underneath suggest (to me anyway) that the machine would have originally been treadle powered – what do think apprentices were for?