speaker sensitivity rating in Watts?

I have a set of Magnat Magnastar speakers (originally purchased in 1988) on which the sensitivity rating is specified as 1.8 W.

There is a photo of the speaker rating here: http://www.magnatmuseum.nl/Index%20overige%20rest%20magnaten%20en%20kloons/magnastar.htm

In all modern examples I can find, including current speakers on Magnat's web site, speaker sensitivity ratings are always expressed in dB. I understand that the dB rating represents the speaker's loudness measured 1 meter in front of the speaker when a 1 Watt signal is applied to the speaker.

My guess is that the 1.8 Watt rating on my speaker is an older (or German-specific) way of measuring the loudness, in which power applied to the speaker is varied to achieve a fixed loudness measurement. If I knew what loudness the measurement was trying to achieve, it would, in theory, be easy to convert to the current measurement system.

Does anyone have any insight about this older way of measuring speaker sensitivity?