There were two different styles of block thermostats and this is covered in a 1958 LINC-MERC Service Bulletin. They are interchangeable. The problem is that the block thermostats are no longer serviced as LINC deleted them in early/mid sixty production. They are not necessary for normal operation. The diverters are.
Read this thread in it's entirety;
http://www.network54.com/Forum/message?forumid=257364&messageid=1093058276
There is complete information contained in past posts on this forum if you are wondering about the purpose of the design.