transit – Transiting the UK with a residence document issued by an European Economic Area country

If I were you I would get a visa.

The document you have is not an EEA family permit, which was the name the UK gave to the documents that it issued like the document you have, but the UK never used that name to denote such documents issued by other countries.

The document you have is not a common-format residence permit. It is certainly a residence permit, but it is not in the common format specified by European Union law.

The document is not a visa of any sort, and it is certainly not a uniform format visa.

Now obviously in the spirit of the transit visa exemptions you ought to be able to transit without a visa. But the document you have does not comply with the letter of these restrictions. It is better to be safe than sorry.