Well, don't everybody answer at once!
Anyhow, watching a video by Kowa suggests to me that the main differences between the TSN 99 and the older TSN 883/4 models are as follows:
1) Better viewing in very low light conditions;
2) Slight increase in magnification;
3) Some enhancements to lens coatings, the use of a better "secondary" lens in conjunction with the "primary" fluorite crystal lens, and the placement of filter threads on the body of the scope itself instead of inside the lens hood--this means that when the lens hood is extended the outermost filter stays shielded by the hood (i.e. stationary) rather than moving with the lens hood and thereby remaining unshielded from solar glare.
So they appear to be pretty close, with enhanced low light viewing and slightly increased magnification.
That said, I'd still love to get a chance to look through one of these TSN-99 scopes.
Edited by saidentary, 07 December 2021 - 09:49 PM.