this has a few issues though:

1: You cant do initial stock takes and initial cost this way-doesn't have the field for the initial cost
2: It requires you select the class every time, but as the class is potentially already attached to the product in products and services, why the need to select it again?

The fastest way I have seen is to run a product report by stock and then update each item there, as it doesnt refresh the screen every time as it does with products and services. A manual refresh every X products saves time between each update.

But as it stands, this is a manual process which I believe could be made simpler by just linking a few fields such as the class and saving hours of work. I have 500 odd products I need to do initial cost and stock on and it will probably take 8-16 hours to do them all at the moment.