Looks like any rear shock, fork internals, pro taper bars, unbreakable levers etc, all of which are OBVIOUSLY manufactured AFTER 30/12/1984, can be deemed as ineligible as well then.....
Question is, how far do we want to take the eligibility rulings?
You gotta remember, that VMX is generally about 'recreating the period' or what ever you want to call it, there are some things that are allowed that were not available back then and somethings that were available that are not allowed now. Its just the way it is as it is impossible to recreate everything exactly how it was.
2 examples of things that are of the correct period that are not allowed are aftermarket watercooled heads in pre78/evo and original travel on certain bikes eg 77 Husky as mentioned above. They are 2 things that although 'period correct', are not allowed in todays racing.
Some things that are allowed today that were not available back then are for example wide pegs, fancy high tech shocks and cartridge emulators in forks among others.
You need to look at parts in 3 categories
1 - OEM parts
2 - aftermarket parts/accessories (usually performance enhancing major components)
3 - consumables (can be OEM or aftermarket)
The Keihin flat slide fits into category 1 (OEM parts) and is a major component from a 1985 model. There has to be a line in the sand and if you were to allow this carby or any other major component from a 1985 model bike you would have to allow every single 1985 model bike into pre 85. Its never going to happen. The rule is there to stop OEM major components being used from 1985 models.
Category 2 (aftermarket parts) any can be used if it can be proved that they were available up to the cut off. In this case 31/12/84. The reason being is that they are not major OEM components from/for a 1985 model bike.
Category 3 (consumables) are like others have said 'free' so you can use what you want. You could even use OEM Honda grips or a clutch cable intended for a 1985 model. Now some may say why are those allowed if they are for a 85 bike but not the carby or forks etc. Well the grips, cables, bars etc fall into 'consumables' so can come from anywhere.
To simplify it even more....
for pre 85 you can use
1- Any 1984 model or older bike/part of bike.
2 - Any after market part that is not already considered a consumable and automatically allowed, if there's proof it was available for purchase up to 31/12/84 (for the Kehine flat slide to be considered legal for pre 85 it would need to fit this requirement as it currently does not meet point 1. above)
3 - Any part from any period/bike that is considered a consumable, although its my personal option that you should try and keep your bike looking correct period where possible so this is where i have a disliking to fat bars but if people want to use them that's their choice.
So really the whole 31/12/84 cut off only relates to aftermarket parts that are not considered consumables.
It's all really simple when you break it down like that.
This is how i understand it, its really easy to follow i think.
Another point, scruitineers are not perfect and can not be expected to know every single little allowable/non allowable thing on a bike/in a class. Sometimes they may make a mistake or oversee something and this is how something may have slipped through in the past when it probably should not have.