54cm Basso with filet brazed seat cluster and lugged at head tube? PO purchased in England early 2000s. Ultegra/Dura Ace 9-speed.
Never seen a Loto like this one. Seems more like a Viper? And original paint is new to me as well.

If that’s a Loto it must be a late one (1998?). All the ones I’ve seen were lugged with a lugged fork and a braze-on seat binder, not the collar type shown here. The seat cluster looks like the one on the 1998 Gap posted by Tuberoaster on p. 2 of this thread. That image is not clear enough to see whether the head tube is lugged or not.
As far as I know, all the Vipers had fillet brazing at the head tube too.

I agree. The head tube is lugged. The fork crown looks like a Viper, as well.

Definitely a 1998 Basso Loto...

Pic form 1998 Basso Catalog

Was the $600 I spent too much? Not SLX, But custom OS?

Only you can answer that question! If I were in the market for another bike I definitely would have considered it at that price, others maybe not. For what it's worth, I recently spent more than that on a frameset from the same period. I'm partial to '90s Italian steel.

just picked up, needs to be fitted and dialed in. It skips gears when you press on pedals in the front smaller chain ring. Very light , first non steel bike I have ridden . It definitely needs a more comfortable seat and maybe higher stem. I'm a wee bit older now 😁

Well, I fell for it. Got the stupid certificate but no answers to my questions (what is the tubing and was it a Collstrop team frame), and not even a reply to the last email I sent. Stardue (whoever they are but apparently owners of Bassobikes) claims I have a 1989 Loto but I don't buy that: no SLX tubing, no internal cable routing and likely a different serial number format (A268). "We'll check our archives thoroughly", ha! Still a nice ride though, nothing like my Viper at all however.

Did a custom DIY colour shift paintjob on a mid 90th Basso Viper - the pre owner did buy the frame kit and did a build with Shimano 600 and DuraAce in the mid 90th.
I found that beauty and went for a complete rebuild with all original parts. I only had to replace the bar tape, brake pads and chain (extended the cassette for my hilly terrain). All bearings and the wheelset needed a service.
Also the drive train needed a complete degrease and polish
Parts:
steel frame (size 58 Italian) "Basso Ultra Light Tubing 18 MCDV6 heat threated" 1785g
steel fork 673g
Group Set Shimano 600 Ultegra 8 Speed.
Breaks 7400 Dura Ace with new Swissstop Black pads
Wheelset Mavic Open CD 4
total weight 9,3kg (without pedals and bottle cages - aim was to reuse all parts not to go for light weight)

My love...

My ‘84 Gap. I love this bike. Frame measures 65x61, components are all Nuovo Record except brakes and levers which are Modolo Speedy. Since the picture I’ve added Clement LCG Tires and a Selle Italia Epoca leather saddle.

Man, I'm really missing my '84 Gap. Hugh, I'm first in line if you ever want to sell it...

Here's my 89 Ascot built from MS tubing. Best riding bike I own (I think?) although a bit heavy.