@Entobiker I know folks have already chimed in and I would agree with points mentioned.
I did assume repaint and thus I did not give any credence to the decal being accurate until you mentioned feeling the ribbing. I am not familiar with the Ishiwata tubing, but they may have used a different amount of, different width, different depth, different layout (straight vs. helical) style of ribs than did others. I thought it might be "TSX" (same as "SLX"/"SPX" except all "TSX" top tube also has helical ribbing with TT & DT ribs extending the entire tube; also has same straight ribs in chainstays; did Ishawata have them?).
With the fork potentially being a replacement it does make it harder to date. That style internal routing was mostly common mid '80s and onward; though it certainly was around in the era during the 70s also (and as previously mentioned on decades earlier era bikes). "SLX" was definitely commercially available by early '84 and found on team and custom frames made in late '83 (most larger builders were incorporating "SLX" frames into the line-ups by '85-'86. If Ishawata, I assume that they (and other tubing makers) came out with their "SLX style" tubing after Columbus did so that would date the frame forward another year or so. I know nothing as to if Columbus made straight fork blades in their early (pre-'90s) tubesets, but I would assume that their steerer tubes still used (on top tube sets) ribs into the '90s. That might also ID the fork quality (I assume if d-o's are Columbus then so is the fork?) or a different tubing brand (my SuperVitus steerer has 6 ribs. Columbus' 5; Ishiwata and others may use another identifiable feature over the other. Note: "TSX" introduced c. '88 so if going with it as being an "SLX" frame I would guess '85 and later; if "TSX", '88 or later. If you can date the introduction of Asia branded tubing similar to "SLX" I think you'll get a pretty good feel that the frame is probably later '80s (assuming replacement fork), or '90s if fork is OE. Not sure about "PRX" but I doubt it is that type.
Regarding seat post size, there are reasons why it could be odd/in between sizes. Do not assume bike shop is correct even if using a post gauge. I see them be wrong more times than I can count on my fingers and toes
First, the lug is out of round your gauge reading will be wrong. Ensure lug is as round as possible before trying to gauge it or test various post sizes. Distortion of the lug can occur due to various reasons (heating, frame contruction, inconsistent reaming, wrong post used, etc.) so addressing it first by reforming it can save time and money in the long run. As to it possibly being an "SPX" frame (27.0mm post) that is difficult to say since a buildervuses whatever gauge tubing they want to given a certain model or client. You could easily have a frame made with just an "SPX" seat tube and use "SLX" (or any type) gauge wherever you wanted; but I doubt (unless custom design) that your frame (if Columbus and stock size build) is "SPX" given its size. It was mentioned that a 55cm frame was the general size where buliders would start using "SP"/"SPX" gauge tubing over "SL"/"SLX" (I guess with "TSX" being extra reinforced along entore DT &TT length that it did not require a heavier thicker walled version). I have had many frames (95% Italian) pass under my butt and through my hands and no "SP"/"SPX" frames sized 57cm c-c (58.5 c-t) and under used anything but "SL" if of the same model quality.
