Woo Hoo. Picked one up for $13 including delivery from ebay.
This flowmeter is designed for use on horizontal 1-1/2" plumbing. The flow rate may be measured between 20 and 100 gallons per minute. A dual gallons per minute and litres per minute scale is printed on both sides.
Looks like this one. Just an example here.
Edited to make info clearer.