I eventually located these using the path https://mydomain.com/jsonapi/product-variations/default.
A call to https://mydomain.com/jsonapi exposes the end-points you can use, based on your product configuration. Mine looked like this:
"product--default": {
"href": "http:\/\/mydomain.com\/jsonapi\/products\/default"
},
"product-attribute": {
"href": "http:\/\/mydomain.com\/jsonapi\/product-attributes"
},
"product-attribute-value--rate": {
"href": "http:\/\/mydomain.com\/jsonapi\/product-attribute-values\/rate"
},
"product-type": {
"href": "http:\/\/mydomain.com\/jsonapi\/product-types"
},
"product-variation--default": {
"href": "http:\/\/mydomain.com\/jsonapi\/product-variations\/default"
},
"product-variation-type": {
"href": "http:\/\/mydomain.com\/jsonapi\/product-variation-types"
}