I haven't read Shaw's book so I don't know how my test will apply to his. Today is our first sunny day since you posted and I just tested my PZ-1.
ISO 100 indicated shutterspeed 90
ISO 200 indicated shutterspeed 180
ISO 400 indicated shutterspeed 350
When I read the greycard with my Zone VI modified Pentax spotmeter my exposures showed I would need to increase the film speed by 1 stop, 100 goes to 200 etc.

Hope this helps you. All I've done is confuse myself:-)