Thanks a lot !
.. and I'm also glad because this confirms what I finally found by myself in parallel by some deductions, which I will try to summarize herebelow in case it might help in some similar situations - and in case nobody would answer so quickly as you ;)
The way I proceeded is as follows: the idea was to try to deduce the missing items by mapping the winning combos on reel disk photos, with starting by the RIGHT and MIDDLE reel disks - whose strips were complete.
(1) I mapped all disks with circles -from center to edge- named A / B / C / D / E which maps the holes (Fig.1/2/3)
(2) I divided the disk in sectors corresponding to the 20 stops, numbered from 1 to 20
(3) I named all the holes with the above convention (e.g. A19 for the hole on circle A and sector 19)
(4) I wrote a C program to determine the number of winning combos (i.e. the number of times a hole on a given circle can match on the 3 disks), the result being in Fig. 6
(5) I deduced from this, with reference of an image from internet (Fig. 4) that the most rare combo (full "A") should correspond to the 3 "Bars" which pays the most (except jackpot that I excluded for my thinking).
(6) On the RIGHT reel disk, there was only one "A", and thanks to the particular A/B/C/D holes on the same stop, which suggested that this could replace a fruit, this likely confirmed that "A" was for the generic "Bar = Bell-Fruit-Gum"
(7) Then I guessed an arbitrary position of the reel "Bar" symbol relatively to the A/B/C/D holes line - as per the black lines on Fig. 1; since the payout lever fit into holes at a horizontal position, this means that A/B/C/D holes alignment being horizontal, the visible symbol position for the player could be
the one of the black line.
(8) It was then easy from this first "Bar" symbol to map the whole RIGHT disk, and then the MIDDLE disk
(9) By repeating the same angle as in (7) for all symbols, this allowed to make the synthesis as per Fig. 5
(10) Finally, the completion for the missing symbols on LEFT reel disk was feasible
, now that letters were associated to symbols, and that angle of (7) was known.
What remains unknown to me is how the payout levers are managed (special shape?) for the two-cherries and for the jackpot. On another hand, since I have only the reel set, I don't know wether this was for a six-levers or a five-levers mech.