Ken here, Montana native living in AZ.
i bought a rat rod project that has a Jag IRS. I’m trying to determine what year and model t of this unit so I can replace pads and rotors. The ID tag is missing, but I have the parts numbers from the calipers: 64321787 & 64321788. attached are some photos of this unit.
any help will be greatly appreciated.

 

Inner disk brakes and 4 coil dampers are the key identifiers. This is now known as the first generation IRS, launched in 1961. The second generation IRS was launched from 1986 on the XJ40, while the third generation IRS took over in 1998 on the new S-Type.

Apart from rare Mk 2s modified for pre-production testing, this was launched with the E-Tyoe in 1961 and continued through to at least 1986, when it was fundamentally redesigned for the XJ40. If you wish to get heavily technical, the first gen IRS in its carrier came in three overall track widths (1270, 1372 and 1473mm) with the narrower in S1 and S2 E-Types, wider on S3 E-Type, S-Type and 420, then wider still on Mk X, 420G, Daimler DS420, all XJ6 and XJ12 of S1, S2, S3 and equivalent Daimlers, as well as XJ-S from launch through to about 1986 also.

Even with all that variability, if you are armed with the calipers and the disk diameter, matching and replacing parts should not be problematic.

Cheers and best wishes,