R-138 passable with a passenger vehicle is actually about 1520-1525 kilometres long, if you add the 1392 km between Dundee and Natashquan river, 15 km in Kegaska, 4 km in Chevery, 17 km in Tête-à-la-Baleine, 15 km between Mutton Bay/Red Bay/La Tabatière, 4 km in Pakuashipi and 72.069 km between Vieux-Fort and QC/NL boundary.
Here is the marker

All of those mini-stretches are signed with R-138 shields, although they do not have direction plates (just plain shields and arrows). Some are even paved, but don't expect to get around without a pickup in winter, though.
Dusting off this really ancient subtopic ...
I'm working on the Quebec part of the Travel Mapping project, and trying to nail down all the existing R-138 segments east of Kegaska. We already have the Tête-à-la-Baleine, Mutton Bay/Red Bay/La Tabatière, and Vieux-Fort/Blanc Sablon segments in the TM database (I drove the last segment in 2011), and already know about the proposed/in-progress new road that would connect the Tête-à-la-Baleine and La Tabatière segments.
About the Chevery segment, Open Street Map and
Mapcarta show a QC 138 segment there, about 4 km long, matching the Transports Quebec/MTQ segments shown in various government online maps. (This segment is west of Harrington Harbour -- some online maps think Chevery is northeast of Harrington Harbour, connected by road to Tête-à-la-Baleine.) And
a 2015 report from a Chevery resident states that Chevery has a 6-km long segment of 138. But the Transports Quebec/MTQ segments in Chevery are identified in government
online maps as provincially-maintained, but with route numbers beginning with 50313 or 50349 rather than with 00138 for the Tête-à-la-Baleine and La Tabatière segments.
OSM, Mapcarta, and other private online maps I have reviewed don't identify any road in or connecting to Pakuashipi as a QC 138 segment. Transports Quebec/MTQ
online maps show the road between the airport/St. Augustin passenger ferry and the Pointe-a-la-Truite main ferry dock (about 7 km long) as provincially-maintained, but similar to Chevery with route segment numbers beginning with 50341 or 50410 rather than 00138. Also, Quebec's 511 system map shows that segment (right now closed by bad weather) with a provincial route marker but no number. (Nothing about the status of the roads in Chevery, Tête-à-la-Baleine, and La Tabatière, not even the confirmation of good conditions on the Vieux-Fort/Blanc-Sablon segment.)
Also,
this undated map indicates that a few km of QC 138 are already complete in La Romaine (and also in Chevery, but not Pakuashipi). This is similar to the Pakuashipi segment, with no unofficial online maps indicating the route designation, and official maps indicating a designation other than 138.
Any info I'm missing to confirm or not QC 138 segments in La Romaine, Chevery, and Pakuashipi, and whether they are signed as such? I've searched for "Romaine", "Chevery" and "Pakuashipi" in the French-language version of the Transports Quebec website (more complete than the English-language version -- English translations of some pages are available on the French site version), and got some results but they don't really answer my question. Not knowing much French, I don't know if I'm missing something on the Transports Quebec or other government websites.