robinjojo, That is an impressive double sestertius ! I remember seeing that coin at a Roma auction last summer . Identifying the under-type on your coin will be difficult because what is clearly visible is the reverse of the host coin, not the obverse.
Piling on with a Postumus overstruck on an Antoninus Pius sestertius: IMHO there are three levels of overstrikes: 1. You can tell it is an overstrike but that is all. 2. You can identify the undertype at least partially. Obviously it is better if you can read more but even telling the ruler is better than nothing. 3. The detail is clear enough that you can identify fully both sides of the undertype and overtype. I consider mine a decent level 2 because of the profile of Pius but I am less certain about the reverse of the Pius. These things are easier to decipher when the obverse was overstruck by the reverse since you can know that which details belong to which strike more easily. On one like mine you have to look for details that do not look like the galley of the Postumus strike. I am not a Pius specialist so I am not familiar with the options.