Strange. I've never seen a clear hatch on a 'Dak', and apart from the thousands of photos, I've seen hundreds of the real thing, and flown in a few, including the cockpit, in the last fifty years. The inside of the hatch normally had the same padded, quilted sound proofing material as the bulkhead and part of the sidewalls, with a fairly substantial seating frame in the fuselage roof, and around the hatch itself.
But, at least you can see into the cockpit with a clear hatch! (Which might be Trumpeter's intention, as an option).