Three Weir Winders

Today we did the 'Panorama' walk from Pateley Bridge to Glasshouses and back along the River Nidd. When we left home the weather looked promising. In the Nidderdale valley, however, the mist came and went, as did the sun about once every 10 minutes and it was rather uninspiring for photography. 
Alongside this the weir on the River Nidd are these three sturdy looking winding contraptions which I can only assume are for adjusting the sluice gates. I looked up (googled) "J. & G. M" that appears on the side of each winder, but could not find anything that obviously related to their origin. The weir itself can be seen in today's extra.
In the process of doing some research, it appears that there are various types of weirs - Sharp Crested and broad crested  to name two. The things you learn whilst researching for blipfoto!