Thats the FRM.
A couple of aftermarket companies made an aftermarket one that takes a gauge. Planet did one.
But be aware some of them alter the position of the reg which may change the 'feel' of the marker when you hold it.
Your best bet for getting hold of one is either classifieds (more than likely someone has one in the bottom of their gearbag, or failing that try Ebay.)
Make sure it works with the Ir3/LCD and is not a newer or older one.