Hi: At the risk of revving up even more dialogue on a topic that's received plenty of coverage already, I thought I'd share this photo and info since it seems to apply to the original question and may be helpful in the future.
I've been working on my '74 TCS for 6 months after rescuing it from 25+ years of "storage." Have not been able drive it yet due to nagging brake system issues, but I finally decided to go through the shifting mechanism. Turns out I have TONS of play at the front shift rod clevis connection, so new bushings are in store. However, when giving things the once over, I found that the 5th gear tang on the external detent setup had apparently been ground off.
At first I thought it was another DAPO "fix," but reading this thread helped me realize (I think) what the deal is with my car. It definitely has strong 5th detent action, so the internal spring and ball are in place. Looking at some other articles, I believe what I may have is a factory expedient used in the very early 5 speeds. My car, 3826, is an early build '74, built in July, '73. Or perhaps they just kept using the 4 speed detent plate until they ran out?
But of course, this still doesn't answer the question of why/when Lotus stopped using the internal setup. As an aside, I found photos of the same setup on a Renault forum where posters were scratching their heads as to why a used "Renault" box had grown some sort of homemade looking mutation.
https://www.aussiefrogs.com/forum/index.php?threads/365-box-mod-from-a-lotus-europa.148492/ Ron