If the slots were there, they wouldn't show clearly in your photo, Paul. But I've added a mark showing roughly where one would be. The other would be hidden by the TOB.
My photo was actually a non-OD 71 Stag box (only one I had handy for photos), but if it originally had OD, it would have been an A-type. I don't know offhand when the slot disappeared, but it could have been when the OD changed.
Or not, since AFAIK the ODs & main housings are fully interchangeable. The transmission in my TR3A has a TR4 A-type (with the big accumulator) mated to a late TR6 main housing (no slots IIRC and much beefier than the TR4 housing) with a TR3 top cover (to keep the dipstick).