On 7/4/2013, Jim Giunta in Bristol, Pennsylvania, USA wrote:
"I bought my MGA 1500 in 1977, no problems other than it's time to do the rockers. .... I haven't run the car for 13 years. .... I topped off the coolant, hand cranked it for a minute or so for lubrication, put in a good battery and she started right up. I ran it for a few minutes and shut it down. Next time I went to start it, the coolant was low so I added some (a lot). Then I realized that the coolant was coming out of the water pump weep hole. .... So I put on a spare pump and went to do this all over again. But to be cautious I checked the thermostat, the pictures are what I found".

This looks a lot like the coagulation problem of mixing OAT and EG coolants (prior page), but there may be a slightly different cause due to the long term storage. With the pot metal thermostat cover on the iron engine block, and a mixed metal thermostat inside, there may have been some galvanic corrosion happening. Difference of electrical potential of dissimilar metals results in a thermocouple capable of creating minute electrical currents. This is the process that can permanently bond a steel screw into an aluminum housing. It can also have strange results with passing the current through a mixed liquid solution. This appears to be another case of coagulation of the components of antifreeze collecting in the highest point of the cooling system. Being inside the thermostat housing has me suspicious that it may be (partly) a result of galvanic action.

This is a classic case of SAD MG. Classic cars really don't like sitting around stationary and neglected.