On 6/24/2017, Dan Barlow in Cheshire England wrote:
"Rebuilding the front suspension and noticed that one of the springs removed had metal blocks. Not sure if they are assisters or something else have you seen these before? It was a US car".

Here in the USA we have JC Whitney (which also used to be Warshawski and Co). These spring boosters have been available for at least 50 years, meaning they are period correct accessory parts for MGA. See here:
www.jcwhitney.com/mr-gasket-coilspring-twist-in-boosters

They are commonly used to increase length of a coil spring that has collapsed some with age. Occasionally someone may use them to increase ride height of the car. Aside from lengthening the spring, they also damp out any motion of one coil of the spring, which has the effect of making the spring stiffer (higher spring rate). In any case you should treat them as poison, chuck the suckers out and replace the tired spring(s).

When replacing springs, be sure new springs have the correct height and spring rate, and that left and right side springs are identical. Since it is difficult to get a new spring that will exactly match an old spring, it is recommended to replace springs in pairs (matched set). See prior page for MGA coil spring specifications.

These days it is common for new replacement springs to NOT conform to the correct OEM specifications. They may often be too long or too stiff, making the car sit too high and screwing up handling of the vehicle. If you get new springs, carefully measure height, wire diameter, and number of active coils. Be prepared to send them back if they are not correct, and find another source with better parts. It may save some time and pains to post a note on a bulletin board to ask if anyone has recent experience with good or bad replacement springs.