Identify that leafspring!

In preparation for the arrival tomorrow of MB Old School 17x8 wheels, I pulled the wheels of my 65 fastback tonight. All of the suspension is new and I was told by the previous owner that it had the Shelby mod control arms with 620 springs. Now the rear, I have no clue. Can anyone identify what the springs are by looking at them? The have the writing "4-1/2 M10" on them. The car has the typical "sag" look to it and I'd like to fix that once I get the new wheels on. Everything is less than 3 years old. Any ideas since I dunno what the leafs are? I suppose I could cut the front a bit, but it's already low and the 17's are gonna be a tight fit. Oh, it does have the Shelby traction bars as well. Thx!
 

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Your springs are definitely NOT original, Shelby or anything else. All 64-73 Mustangs had 4-leaf springs, even the 428SCJ, BOSS429, etc. They also had nylon pads between the leaves, not pierced through them like yours. The 'under-ride' bars were factory equipment on the 1966 GT 350. However, in 1966 they had cast steel rear brackets, yours are welded steel, the later aftermarket type.
 
4-1/2 leaf mid (MID, not M10) eyes (sag built in) for lowering over stock. Possibly Grab-a tracks. I just got a set of 4 leaf mids, the ID writing looks very similar to mine. Extra 1/2 spring is to control wheel hop, I wouldn't think you would need those and the under riders.
 
Identifing marks will probably depend on manufacturer. Non stock springs that are designed to be 1" lowering will lower do just that (or close to) regardless of the spring rate. There are some lower than 620# rate springs on the market. I have 600# springs, I still haven't driven on them. Some say 620's are too stiff and rough for street driving.

Cutting a spring will increase the spring rate/stiffness, so I wouldn't recommend it unless you find the rate on yours is lower. Can you post a picture of your coil springs? If there is an ID, it will probably be written on the outside of one of the coils.