I believe they went to red-headed dipsticks for visibility. It appears TH350s had them too.
Here's a listing for a supposed low-mile 84 dark blue with a red trans dipstick listed as a 3 speed.
And another one with a red dipstick. Window sticker says MX1 (3 speed).https://classiccars.com/listings/vi...e-carlo-for-sale-in-alsip-illinois-60803
And a 3rd. Dash gage says it's a 3 speed.https://classiccars.com/listings/vi...e-carlo-for-sale-in-ft-worth-texas-76137
Without engineering documents, it's hard to predict/determine when and where, and mostly why, they would do a mid-year production change. Sometimes they knew a change was coming, but can't pinpoint when it will happen in the production year. Could have been waiting on EPA or some other alphabet group to sign off on, or parts suppliers to ramp up for supply.
I've always tried to figure how they determined there were only (3) 84's made with the 4R, potentially. I could be wrong, but I believe it to be a wives tale. It's like the old "X were made with T-tops", etc. GM didn't keep that detail of records. Closest they keep to drivetrain is V8 with Auto transmission. If there was a designator in the VIN, then they kept records of that, so for example, they can tell you how many blue 84 Monte SS cars were produced with the L69 engine and auto transmission, but that was about it. And even if you found one...so what? It's not like they're a GNX. Starting in 85 it was standard equipment.
It is true that the factory engineering documentation in late 1983 had designated a 200-4R with 3.73 for the L69 as "interim availability" (read- coming later in the production year), with the exception of California. California wasn't slated for getting a Monte SS (or any El Camino in any state for that matter) with a TH200-4R in late 84 model year.