Fortunately an easy one to answer
Luckily, what we could call my first milk regarding F1 was the excellent Gerard Flocon's book
LES GRANDS PRIX based on 1980 season.
Well, the thing is easy: JS11 throughout 1979, JS11/15 throughout 1980. E basta!
Don't be fooled by the lowered fuel tank lower, the rims, or even the engine cover.
What REALLY differenciates both cars is the season... and something more.
Let's see:
-Yes different rear axle. Basically the rear suspension concept is much the same, but brakes went outside, fars from differential, on the 11/15. That's it, inside the wheels. In fact, All the suspension is the same, simply the axles are a bit longer and the discs, located outside the uprights are covered by a different rim.
-Different sidepods. They end before rear wheels, a la Lotus 78, leaving the exhausts uncovered. Longer, less curved, and covering flattened layout of exhausts on teh 11/15. Similar to FW07. All that, seen from underneath: because, from above, all looks much the same.
-Yes engine cover, it leaves seen the DFV camcovers on the 11, covered on the 11/15. There's one exception: in 1980 South african GP, cars started with JS11's engine covers. That might lead to the wrong idea that JS11 raced sometimes in 1980 (there was one as a T car in Argentina though)
-And the most important feature. The nose. Well, the area where the 25 or 26 number goes,
is clearly seen in ALL 1980 pics as flush, same level that the suspension fairings external edges. This is a very characteristical feature that gives the car a very sharp look seen from the front and that accentuates the low nose effect, and somethinh I particularly like, case you hadn't noticed
Obviously that's not like that on the Tamiya model, where central section is clearly higher than external, and has not an easy fix (chassis is tightly underneath), because also the nose cone is too high, the suspension fairings too thick and they lack an inclined lower face. This... for the 1980 car, the JS11/15. Because for the JS11 (1979) I'd say it's more or less accurate. I have the suspicion that the car was "flattened" around mid 1979,
but I am not sure, I'll have to investigate. The Heller 1/12 car has also this, but if you fill the longitudinal sinks that separate center from outsides, the think is fixed.
-And now, accessorial features:
-Wings: different config, with very slim ones used at South Africa (and maybe some others high speed tracks), high downforce ones for Monaco and Brands, and generally two element ones. The intricated wing supports were similar to the 1979 ones, but simpler.
Yes No front wings in quite a lot of tracks, I can check if needed.
-Fuel tank cover: yes, was changed at French GP and kept. Not only is lower and rounded: also the white section of cockpit sides, that is a seen surface of the fuel tank on the JS11 and JS11/15 till before France, with the low cover this section is included in the cockpit bodywork... as usual in those days' cars.
-Rims... Varied. Black 13" three spoke (I think) dymag fronts until France, Chrome 15" of varied brands since Brands, after Jones' 15" french victory. First Gotti, then Campagnollo (or the contrary, I should check) and generally 6 spoke.
Rear ones changed less: 13" always, diffferent brands and I think that always chrome, but might be black at season start...
-Last but not least, Sidepod plip ups, they were bigger or smaller depending on downforce requirements. That was made with rather unelegant aluminum palte add-ons, more or less acceptable for upper surface, uglily riveted seen from underneath. Also they sometimes had small profiles that came from the underneath of sidepod, sometimes not. External and inner fins also changed sizes.
-More details: Yes skirts were different, but hard to spot! and most noticeably, the swap from rounded
GITANES to italic
GITANES on sidepods. Rest of GITANES on the car remained unchanged (!!!)
Hope this all helps...
ED: while I was writting this post, good old JP wrote his: don't be fouled by his nick or the fact of being french
I am more acurate, it's not me or my memory. IOt's just Flocon's book and GPI
