An-12U (1962) Its wing was equiped the simple flaps instead of 2 slit flaps, providing them to be blowed by the DK 1-26 engines' air stream. In the same way the engines gave air flow on stabilizers and other ruling surfaces as well.
An-12D (1965) Modification with hermetic cargo compartment and increased fuselage's diameter. Cargo cabin's height was 3,45м.. They expected that under take-off weight 83 t. the plane will be able to carry 20 t. of cargo up to 3750 km .Its airborne troops' capacity was increased up to 100 persons. The engines were AI-20 DK (5180 ehp.). The An-12D further development became -
An-12D-UPS, similar to the An-12U, but already with three additional turbo-compressors , two were located in the zooming wing's backsides, and one one more in the keel's basis.
An-12SN (special purpose plane, 1963) with increased size of the cargo compartment in order to carry T-54 tank (37,2 t) The same AI-20DK engines plus RD-9 jet engine (thrust=3800 кg) in its tail.
There was another project (1969) using a sweptback wing and turbofan D-36 engines (6,5 t) but its marking/model is unknown to me.
An-40 (1964) Outwardly looked like An-12 with a few important particularities. Power installation consisted of 4 AI-30 (5500 HP) and 4 runaway-brake jet engines RD 36-35 (2550 kg). These engines with short resource were created for Yak-36 VTOL and were used in the project for reduction of running distance. Combined chassis with skis allowed (theoretically) to land/take-off from small, unpaved and snowy airfields.
All these changes outwardly were not so noticeable, basically. That's why there were almost nothing published in the aviation-related media, I think.
The majority of these specified modifications aimed to improve wing's ascensional power characteristic, it was all about BOUNDARY LAYER MANAGEMENT=Управление Пограничным Слоем=УПС/UPS (they use "blown flaps" in English), hope you will understand it correctly, as it is hard to translate such specific aerodynamic science things.
Antonio, hope everything would be useful for you, as I have no drawings yet - though I've ordered some books. That's all I have now.
Sources:E.BUCKIY ,S.MOROZ "Transport plane Antonov An-12"
N.YAKUBOVICH "An-12. Fourty years in formation"
A.SOVENKO "An-12 portrait"