Thanks for the advice :(, but what if my watch (for example) would not had such a label inside the caseback, since Your attached image cannot be seen? :). After all, at the watchmaker check, my watch had the inside label, but I really don't understand why so much cautions from You to CLEARLY present this straight solution, since it is already a very rare watch and the same thing it is written in the manual, but not very clear for everybody!
I would have been much more thankful if somebody would have clearly told me the below-presented simple solution I've asked for ( in my post above about ABC simultaneous buttons combination for RC 4000/4400).
ANYWAY, below is presented the solution I was asking for - after all, and as I suspected, the solution IS THE ONE presented in the manual of the watch, tried and verified during the "revival" of this vintage and outstanding watch with my watchmaker:
I was with my watch (Seiko RC 4400 PC Datagraph) at my excellent watchmaker, so - after the proper battery replacement - it is now working like a charm :).
If I will have some time I will post some more photos of this NOS RC-4400 and/or I will make a full review with (hi)story :).
Some advice for other users:
If the battery was removed for a long time, after battery replacement the watch may display a "garbled" screen and/or will sound continuously.Thus, to make it crystal clear for all current or future users of such watches: The "magic" solution to this problem is to press SIMULTANEOUSLY the A-B-C buttons combination for few seconds (3-5 s), that will do an AC (All Clear) reset of the watch, as it is mentioned in the manual page shown in the picture below:So, the regular ALL CLEAR reset for the data will also perform an AC reset if the watch may function strangely AFTER the battery replacement!It's a kind of "magic" ...
Hope this will help also other users/owners of these outstanding vintage watches,
Best regards,