Dave,
If you are referring to the 'ladder' leading to the right-side crew door behind the cockpit, I think you'll find it is just a lightweight aluminum ladder ( or half of one of those fancy multi-ladder things that's carried onboard. If you mean the orange drop-down door on the right, I suspect it's just the forward cargo/baggage door opened for crew access to the systems.
There was no factory airstair for the Dougies as that was an idea that came along after the DC production run was well-established. The only ones I've seen were custom ones installed at the 'normal' left-rear exit on N100J (DC-4) and N100J (DC-6) owned by INTER-AMERICAN MINERALS CORP a.k.a Canadian Javelin, a mineral exploration company. Both were fancy executive conversions (including gilt and marble tables and gold-plated washroom faucets) that flew around the North Atlantic and to the Caribbean in the 60's. The owner wanted an airstair so they could park at FBO's that wouldn't have a DC-sized mobile stair (or reportedly for quick getaways).
Since it has been done, it's possible that others tried it but I'd be surprised if a Tanker would be equipped as the system was fairly weighty.
