Exactly. As I pointed out earlier, the main difference between the S-55 & S-58 is the powerplant. The S-55 is piston-engined & the S-58 is gas-turbine powered.
I think that may be too much of a generalisation as I think the S-58s were radial powered too (See the H-34 above...).
The other main difference with the S-55 and S-58 is the rear fuselage/tail boom, high set on the 55 (trying very hard not to say Whirlwind!) and large and chunky on the 58 (trying very hard not to say Wessex!)...
Until today I hadn't noticed how similar they were...