Indeed, the answer to your primary question can be found on
page 4 of the old forum's Crystals and Gaskets thread.
The story actually starts on
page 2 of the thread (you'll be well recommended to read the entire thing) with this post:
- YemaSpationauteIII-Crystals&Gaskets-Feb2014-Post.jpg (138.84 KiB) Viewed 6713 times
It continued a couple of months later, in April 2014, (on
page 4 of the Crystals and Gaskets thread with above quoted):
- rsz_yemaspationauteiii-crystals&gaskets-april2014-post-1.jpg (91.4 KiB) Viewed 6709 times
- YemaSpationauteIII-Crystals&Gaskets-April2014-Post-2.jpg (82.81 KiB) Viewed 6709 times
In the original thread, I've posted a sequence of 6 photos, showing the increasing levels of distortion caused by using that flat bottomed replacement crystal, as the watch approaches the horizontal angle. You can view them there.
- YemaSpationauteIII-Crystals&Gaskets-April2014-Post-3.jpg (124.09 KiB) Viewed 6709 times
That said, it's a very cheap and acceptable solution as a replacement for a badly scratched crystal. Unless you happen to spend a lot of time looking at your watch sideways on.
Even today,
Cousins p/n DF250CMH305 costs a mere £1.95. This is
exactly the same 30.5mm flat bottomed mineral glass generic replacement that UK eBay seller
watchfan700uk would have you pay
£14.99 for per the link you included in your PM:
https://www.ebay.co.uk/itm/123832637882 and
jfield wrote: ↑Sat Jul 10, 2021 5:37 amThere are some I found on eBay (shipped from UK) with part #459 ....
.... with that
(completely irrelevant) p/n #459 reference. Many of the substitute replacement crystals listed by that eBay seller (real name Stuart Tomanek) are nothing more than Cousin's cheap generic mineral glass crystals which he presumably repackages. I strongly suspect he got his inspiration for many of the Seiko 7Axx substitute replacements (and the Spationaute III) by studying my findings in the Crystals and Gaskets thread.
Regarding replacement crystal gaskets, sorry to reiterate, but if you've taken the trouble to read through that thread, you'll know that I personally advocate
not replacing them, unless absolutely necessary and re-using the original OEM gasket whenever possible. I can't remember having replaced the crystal gasket in any of my Spationaute III's, but may quite possibly have done so in my original 'parts donor', that subsequently became my 'bitza' mission flags dial watch.
- rsz_yemaspationauteiii-mine-p1000854.jpg (210.13 KiB) Viewed 6679 times
Damn !! There's so much useful information in that Crystals and Gaskets thread - and an 'aide memoire' on page 2 !!
Incidentally, that much loved 'bitza' Spationaute III (my very first) with the desirable 'mission flags' dial later benefited from fitting the correct original crystal and gasket (courtesy of a scrap Yema L5 OQ4 6 Ana-digi Yachtingraf*) and by subsequently replacing the later polished hands (wrong for this variant) with the correct earlier silver painted set.
*The L5 OQ6 6 Ana-digi Spationaute, which shares the same case as the L5 Yachtingraf also use the same crystal.
As you can see, I mounted the crystal on a rubber mandrel, held in the chuck of a mini-drill, using double-sided tape - and then spun it up applying various grades of Wet and Dry paper. This might seem the logical way to go about it (or at least it did to me, at the time), because the rotation should help achieve a consistent profile. It does. But it also generates a fair amount of heat, which causes what's best described as 'particle flow' and every subsequent attempt at sanding with finer grade papers results in clouding. 
He eventually managed to reach the required 50-post mark, to legitimately advertise them. Since then he's received some very positive reviews of his work.
