At this time in history, I suspect that you might find the OEM-Production Imperial shocks from an obsolete parts supplier as
@mobileparts or possibly an eBay seller. I find it interesting that a '73 Imperial shock might have a different part number than a similar C-body shock. But considering that the particular part number would also include the related upper mount bushing, perhaps it was a different one than the normal C-body shock used, although the shocks would be the same, otherwise.
On the replacement side of things, Monroe lists their current OESpectrum shock for "1973 Chrysler, Imperial" as the "5811" part number. Seems like that was the same part number that also went with their prior "RadialMatic" front shock? The RadialMatic replaced the prior Super 500" as their HD shock, when all shocks had to be "radial tuned" for best results?
End result is that what might have been two different OEM part numbers have been combined into one replacement shock number in the aftermarket. In the "Buyers Guide" section at
www.rockauto.com , those Monroe front shocks are C-body '65-'73 and Imperial from '657-'73, which seems appropriate to me. Putting in the model and year at
www.monroe.com yields the 5811 OESpectrum front shocks and MonroeMatic rear shocks. OR rear Load-Leveler or air shocks at the rear, too. So that's the current state of affairs, it seems, with resoect to Monroe.
In another forum, a few decades ago, several posters back then lauded the KYBs are being the only real alternative on C-body shocks. They claimed that other C-body shocks seemed to have B-body valving in them, which was felt to be weaker than the OEM C-body valving (which might have been fueled by the fact the rear shocks from Monroe also include '75-'78 Cordobas in the buyers' guide listing?). They claimed the KYBs were stiff enough to handle the front end weight, fwiw. Or perhaps they were stiffer, which is what the poster desired in the first place? That was the first time I'd ever heard of using KYB shocks on a C-body.
Just some thoughts,
CBODY67