NiberBikes you can find an adjustable 500 carbon pile Load Tester at Harbor Freight for $49.99 on sale. Search the HARBOR FREIGHT site with the words BATTERY TESTER.
A 500 CCA battery should be tested at 1/2 its CCA rating or 250 amps for 15 seconds. The voltage should not droop to less than 9.6 at 68F 20C
You were testing batteries with a genuine adjustable load tester rather than a fixed "potato grater" so common at retail outlets. Same with the FREAS hydrometer. A lot of people who have not worked with a Freas have no idea how much EASIER the Freas is to read. You gave me a chuckle with your care of the instrument - that is how much care I give to my Freas to this day.
OK, I'll throw out a bone with regard to testing golf car batteries.
220 amperes for THIRTY SECONDS.
Voltage should not drop to less than 4.8 volts
L16's to 440 amperes, for 30 seconds. Look into the cells using safety glasses immediately after the test. Cells must not emit "smoke".
Drawing 440 amperes at at 4.8 volts is no easy chore.