I thought HP5 was also made up with two grain types...One fast one slow. Mixed together in the same emulsion, I do not know... and what type of salt? Bromide? Iodide?
'Grain type' is a problematic term as it smashes a number of parameters all into one, such as:
* Main chemical composition - film mostly uses silver bromide with a small fraction silver iodide
* Dopants, sensitizing dyes, acutance dyes etc. that are also part of the chemical makeup of the grain, or at least closely associated with it.
* Grain size
* Grain geometry/shape/aspect ratio
Probably some others, too.
When it comes to e.g. grain size and/or shape, they're not necessarily distinct, but generally within a certain bandwidth of a parameter. E.g. grain sizes ranging from x to y microns. So grain 'type' in itself is not necessarily clearly defined - or at least not universally so.
Then there's the distinction between coating layers and constituents of an emulsion. We often bunch it all together in the amateur domain, but there's of course a difference between one single coating layer that contains a mixture of different grain types vs. multiple coating layers, each with their own grain type(s).
It must matter. I'm sure it would have been tested. I wonder which layer is made up of silver iodide...
It's logical that the top layer would be the faster layer. You'd want to limit attenuation of incoming light for that layer, specifically. Putting a fast layer at the bottom is somewhat antithetic, especially in a monochrome film package.
I doubt any emulsion is pure silver iodide btw. I'd expect it to be a bromide-iodide mix; the ratio may very well differ, and as pointed out above, it's just one factor among many that affects emulsion performance.
Maybe
@Lachlan Young could comment; if anyone is aware of Harman-specific emulsion makeups, it'd be him.