Bill,
I think most of the fleet came from the factory equipped with blocks made by Merriman Brothers. The company is no longer in business. If you are a member of the INA (if not consider joining) go to "Stores", then select "Replacement Blocks" then select "Nonsuch 30 Replacement Blocks" The blocks with working loads are listed along with the Garhauer Minimum Equivalent. If you have a Schaefer and want to stick with that brand I am sure they can tell you the equivalent to the Garhauer.
I assume you are looking for a turning block near the base of the mast that is attached to the mast collar but not the halyard block. One used for reef lines, topping lift or choker. A picture of your current blocks would be helpful.
Mark Powers