picked this up today,any ideas on what if was used for?

Fixed Pin Spanner Wrench, most likely a machine tool application, lathe or milling machine. Here is a link to a Billings Catalog.     https://archive.org/details/BillingsWrenchesCatalogW482/page/n25

I have a bunch of those spanner wrenches in fixed sizes and adjustables. I mostly used mine for taking hydraulic cylinders  apart and reassembly.

thanks for helping

The sort of nut/collar that's got holes around the edge for the pin, that's close enough in diameter to the curve of the spanner so it will work.