Just to follow up on this...I contacted TacStar's technical support to ask if they have a mag extension that will fit an SPR453/MP153. Their reply was:
Tacstar does not offer a sidesaddle for that model.
When I responded that once again, I"m looking for a magazine extension and that it has been reported that either the one for the 870 or the 1100 works, they replied with:
Our answer is still that we have no accessories to fit that model.
So..officially, they have no supported product. I'd still be interested to see which part number people have made work on this gun with minor modification.