…is there a seam in historic canvases or is it impossible to see them ?

One of the reasons for using patterned canvases by the Old Masters is the width of these textiles. No or less seams were needed for the large paintings (literature sources)

I did a pilot and joined the selvedges of the reconstruction e.i. I joined each weft on both sides…visible because the selfedges of my textiles being irregular not because of being it a seam….