OK RT, Turns out you were right!  No circlip.  The 5th gear clearance (large gear next to the tiny pressed-on 2nd gear) is simply located by press work.  To remove 2nd gear I was able to get in with a small scale bearing separator and then pulled it off.  But it was tight - creaked and popped about 10 times before freedom.  And in pulling off using the separator, I did unfortunately, just barely, "broach" the 5th gear journal so will be getting another main shaft after all.  To rebuild, I will press back 2nd gear to a shimmed dim - approx 2-3 mil (5mil  didn't fit).
Regards  Joe

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That particular ebay shaft looks like it was dipped in acid to clean it up.  What do you guys think?
Joe

Looks like is was vapor blasted

That's good. Thought i had it right but ya had me doubting haha. You say--if i'm understanding--you'll press up to 2-3 mm. Is that clearance between big gear & pressed on gear. The more end clearance you have the less dog engagement the sliding gear will have if it tries to push the gear away sideways. Good dogs & dog slots & doubt it'd matter. Too much clearance [gear not on far enough] & shaft end clearance to the bearing in the left case might be a problem. Very much doubt it'd ever get to that though--just in theory. I had a mates quad YTM225 or 250 with the same pressed gear & it moved sideways & spun so stopped driving. Nightmare, hate working on those. Had a warranty claim on an almost new yamy v90 scoots in the "70's where that gear let go. You'd think they could come up with a better plan--like a normal spline & circlip.

Well the 2nd gear was definitely an interference fit.  Curious about mfg tolerances on old and replacement parts.  Also regarding when I said I accidentally "broached" my main shaft journal next to the spline (where 5th gear spins), it's not the case.  Shaft was made that way...guessing by mistake. Attached is pic of mine where sublte broach marks show in line with the splined broach.  Some NOS shafts on ebay are much worse...maybe original parts rejects? There is no good reason for all this but I will reuse my shaft...assuming the diameter is still good.