The GMH engine number is a far better indication as they were stamped at the assembly plant in the days prior to going into a car. The cast date and engine completion dates both go on in the US ou Canada. We have seen engines with the same cast date as other HK engines with a HT era GMH engine number.

Originals are worth a mint too especially those specific to later GTS327 and HT-G 1837M engines.

Yes, that is where the McKinnon engine plant put the cast date on SBC blocks.

Baffles for oil control.

Not quite like that or there would have been manual 307's left over too not just autos. The 253 was meant to be released with Monaro in HK but the first one wasn't fired until August 1968. The last HT 307's are 7/68 cast, so putting 2 and 2 together GMH knew what they needed for HT and had to order them prior to the end of the 1968 style 307 engine probably when they knew the Holden V8 wouldn't be ready. They only needed autos as the 253 manual was right to go at HT release. It is possible that 307 demand dropped off a bit in 1969 but I doubt they'd have had many hundreds of 307 auto combos left and no manuals and then used them in HT, but possible I guess.

As for batches yes they certainly had 4 x distinct GTS327 engine assembly codes that we assume were for maybe the original intended 500 or so, but demand saw another 2 x type 1 style assembly dates prior to the type 2 327. Type 2 vary a lot but that is probably how McKinnon did them. The 307's used in the first week of HK GTS volume production are all around 7/67 cast so GMH had certainly got a heap in initially for the start of HK but used limited numbers until Monaro and Brougham saw their use increase greatly. Date codes still are late 67 and earlier 68 until late in 1968 though indicating large batches of engines. HQ used the same 350 in manual applications until the last manuals in later HQ so those were certainly stockpiled. Not sure about HT-G 350's, they are so rare compared to HK V8 engines that it is hard to get enough samples to study.