This set comprises of 8 parts, 4 outer rings and 4 inner rings. The inner rings are half etched to take .5mm acetate window glass. The outer rings have a half etched notch to accept the pin on the inner ring, so the inner window, when made up can revolve. You will be required to open out your existing window hole to 25mm.

The rings are best soldered into position, glue will work also, (but not as well).
 
Please see step 89-99 of the Brass tank locomotive body kit instructions for the fitment of this product.
https://www.swiftsixteen.co.uk/_files/ugd/2ea7fa_a34e0a2482124381af0bf7d6a16c3c85.pdf