Anybody recognize these 340 pistons?

You need to measure the compression height. The compression height of a 10.5:1 340 piston is about 1.84". Compression height is the distance from the top of the piston to the centerline of the pin. Measure the distance from the top of the piston to the top of the pin bore. It should be about 1.350", give or take a few thousandths. If you're wondering how I came up with this number, it's the compression height (1.84) minus half the diameter of the pin (.984/2 = .492)

As to why the piston were down in the hole - that is common on stock 340s. The block heights are always off by as much as .020".

I would not run cast pistons if you are planning on boost!