As your car is 1977 the ratio should be 3:31, if in doubt count the teeth on the CWP and do the maths. The 3:31 axle was introduced in October 1973 Ch R7659 [source Ken Hill]
Do not loose any of the assembly bolts as [some/all?] 3:31 Salisbury axles used 'semi' metricated fixings, i.e metric threads but SAE heads, you can do a quick check with the bolts holding the diff cover - 9/16 AF spanner but M8 thread. Likewise the CW bolts are M10x1.5 and not 3/8 UNC but still have an SAE bolt head!
The 3:31 axle is in effect the next series on from the 3:58 axles, visually similar but different. The 3:58 axles were fully imperial, a different overall length of nose, a smaller diameter pinion shaft and a set [thankfully] without a crush washer. Later 3:31 7HAs may be fully metricated - I don't know.
Were you to find a Reliant Sabre/Scimitar with a 7HA it would be of the earlier 3:58 style of construction.

Last edited by Horror; 14/09/15 05:52 PM. Reason: typo