1970 heater box stamp numbers question

[gtx6970]

Guys,
I'm doing a bit of research on the 1970 heater box stamp.

Basically I'm  trying to narrow down the changover date , build plant and/or built for USA or Canada markets.

I have both 2936680 and 2936681
What I've found and been told so far is,,,,,
680 so far I think is US - built after Jan 1970
681 is US before Jan 1970 ( also used on Canadian market vehicles ) But is it all thru the model year on Canadian market cars?

Also, so far I have not found any differences between the 2

Anyone with there original heater box in place , all you need do is get me the number on the box and the cars SBD and if it's built for US or export if known.

TIA

[70 Charger RT]

I'll have to check, but I have the original heaterbox (non-AC) installed in a canadian sales bank Charger RT with a build date of June 1, 1970.  I'll get back to you.