I've seen this a few times on late 1500s (but not all) It's normally secured to one of the down pipe to manofold nuts via a small egg shaped plate with 2 holes in it, a 5/16" hole for the stud/nut and a pinhole for the spring.
However, since ALL these late cars had concentric return springs on the throttle butterfly spindles, the extra spring is largely redundant! I'll check this when I see you on Thursday and investigate the fast idle.
A cautionary tale! A couple of months ago, I was asked to repair a badly split bulkhead on the well known (in club circles) 1500HL UGH SLOW. Which I duly did and added an auto type bulkhead stiffener bracket as a precaution. Having completed the immediate task, upon testing, the reason for the excessive spitting became obvious, the throttle was so stiff, it could hardly be moved!
This was caused by a combination of factors, mainly a maladjusted throttle cable and linkage which ruined the quadrant operation, but also, someone had fitted a much heavier duty spring in this location. The combination of the 2 faults made the car almost undriveable, but once i'd binned the spring and set up the cable and linkages correctly it was fine, with a free and light throttle operation.
2 door '73 Toledo with Vauxhall Carlton 2.0 8v engine OWF 797M (The Carledo)
'78 Sprint Auto with Vauxhall Omega 2.2 16v engine EGP 247T (The Dolomega)
'91 Cavalier 2ltr 8v auto
'95 Cavalier 2ltr 16v auto
Spectrum Auto Services, Servicing, Repairs, MOT prep. Apprentice served Triumph Specialist for 45 years and home of Maverick Triumph.PM for more info or quotes.