US 401 southern part 1933 Official

US 401 northern part 1933 Official

US 15 southern part 1935 Official

US 15 northern part 1935 Official

Adjustments: In 1935, US 15 was placed on its crossing of Whites Mill Pond north of Sumter leaving behind today's S-43-46 and S-43-325.
About 1950, US 15 was given a straighter approach and a new crossing of the Great Pee Dee River on the Marlboro County side. The old route is today's S-35-167 and the E-W portion is abndoned routing. On the Darlington County side, the old route is the private portions of Bradshaw and Burlington Sts outside Society Hill. The piers to the original bridge are still standing just off S-35-167.

US 15 original Pee Dee River approach 1943 Marlboro County

US 15 modern Pee Dee River approach 1953 Marlboro County

In 1951, US 15 was extended south replacing to US 17-21 Pocotaligo.
In 1952, US 15 was truncated back to SC 63 Walterboro, as US 17 ALT was sent to Yemassee and Pocotaligo instead. See comments way below.

US 17 Walterboro to Pocotaligo 1950 Official

US 15 Walterboro to Pocotaligo 1951 Official

US 17 ALT Walterboro to Pocotaligo 1952 Official

In 1952, US 15 was given an eastern bypass around Bennettsville, leaving behind US 15 Business (today this is SC 385 and SC 385 CONN).

US 15 through Bennettsville 1950 Marlboro County

US 15 bypasses Bennettsville 1953 Marlboro County

In 1964, US 15 was rerouted to avoid central Sumter. Originally using Main St, US 15 was routed around to the east on new alignment Lafayette Rd as it runs today. Main St north of downtown became a US 15-521 CONN (now S-43-5) while south of downtown Manning Rd remained part of US 521 (now S-43-152).

US 15 into central Sumter 1963 Sumter County

US 15 removed from Main St. 1974 Sumter County

In 1968, US 15 was given an eastern bypass of Hartsville, leaving behind US 15 Business.

US 15 through Hartsville 1966 Darlington County

US 15 bypasses Hartsville 1968 Darlington County

In 1987, US 15 was put onto I-95 between Exit 97 and Exit 102, over Lake Marion. The US 15-301 bridge over Lake Marion has been closed to traffic ever since. The road south of Lake Marion became S-38-430 and US 15/301-SC 6 CONN. The approach to I-95 north of the lake had been S-14-400 and the route connecting US 15-301 to I-95 Exit 97 had previously been I-95/US 301 CONN.

US 15 parallel to I-95 1986 Orangeburg County

US 15 moved onto I-95 over Lake Marion 1996 Orangeburg County

Between 2006-08, US 15 was rerouted to meet SC 34-403 at the same intersection when the curved piece between SC 34 and SC 403 was removed.
Improvements: Upon designation, US 15 was fully paved.
The first piece of US 15 to be 4-laned was the first few miles north out of Walterboro, in 1951 or 1952.
The Bennettsville bypass was 4-laned in 1958.
The bypass of downtown Sumter was 4-laned upon opening in 1964 (US 521 to US 76-378 Bypass).
In 1970, US 15 was 4-laned from US 521 south to S-43-251.
When US 15 was moved onto I-95 and the US 301 CONN in 1987 it became multilane.
In 2007, US 15 was multilaned from I-20 to Bishopville.
Comment: US 15 is one of South Carolina's underused routes, as it parallels I-95 for a good stretch and is not that far away from I-95 when it doesn't.
The Hartsville Bypass, while 2-lane does have an interchange at its south end and also with SC 151 Business.
US 15 is poorly posted in the Santee area
So. Did US 15 really exist south of Walterboro? The only map evidence so far is the 1951 official showing it. But there is another piece of evidence...check out the post cards below:

These first two show Lane's as first being located on US 17, then being located on US 15-21 in Yemassee.

US 17 is the location of Lane's Tourist Court in Yemassee 1940s post card

US 15 and US 21 are the location of Lane's 1950s post card

These next two show Lane's in Yemassee on US 15-17 ALT-21 (I have yet to definitively located where this was) and US 15-17 ALT at their Walterboro location (a little southwest of SC 303 jct).

US 15-17 ALT-21 is the location of Lane's Tourist Court in Yemassee 1950s post card

US 15 and US 17 ALT are the location of Lane's in Walterboro at the SCL 1950s post card

There is insufficient county map evidence to accept or refute absolutely whether US 15 was extended south or not.
The March 1950 Colleton County map shows US 17 still. The 1953 Colleton County map, does not offer insight except that the map shows there were revisions to the state highway system March 1952, and Sept 1953 (no idea what the revisions were though).
The Dec 8, 1951 Beaufort County map shows US 17 ALT (previous revision was 1949). The October 1951 Hampton County map shows 17 ALT (this is a base map so recent revision dates not shown)
Secondary evidence shows the December 1950 Dorchester County map with SC 64 instead of US 17-A and the next revision was 1954. I don't have Berkeley or Georgetown County maps in this date range.

So my theory at this point is that US 15 was at least openly discussed as being extended enough that a business had its post cards reflect the change. But I need to find some photo evidence or SCDOT records to definitively say there is something beyond circumstantial evidence.