what type of wire is this?
It's effectively a SE Style U cable, even though they didn't call it that back in the day...

From the information and photo given, we can tell that this is a Type SE (Service Entrance) cable with two type RH insulated 6AWG aluminum conductors, a 6AWG concentric bare neutral or grounding conductor, and a cloth jacket.  Modern cables with this construction would be marked as Style U for an Unarmored type of cable (vs. Style R for a tape Reinforced cable construction, often with four wires instead of three), but your cable apparently predates that marking requirement.

...and it's no longer Code for more reasons than one

While this cable was legal to use at the time it was installed, it no longer meets NEC requirements for a range branch circuit for multiple reasons.  First off, a new installation would require a four-wire (hot, hot, neutral, ground) branch circuit for the range, in order to avoid the shock hazard an open branch neutral poses in a 3-wire range or dryer circuit where the appliance basically "bootlegs" its grounding path from the neutral.

Furthermore, the cable appears to predate the introduction of AA-8000 aluminum alloy conductors, which means that it's made from AA-1350 aka EC aluminum, which is no longer legal for SE Style U cables.  However, a SE cable that does not have a Style marking, while no longer listable by UL, is permitted to be made from AA-1350 as the relevant Code section (NEC 310.106(B)) only applies to SE cables marked as Style U or Style R.

The simplest and most broadly applicable fix for this, provided you can get a 50A GFCI breaker for your panel and have the space available there to install it, would be to replace the existing NEMA 10-50R with an Al/Cu, 75&deg;C rated NEMA 14-50R wired H-H-N with the ground left unconnected, mark the faceplate for the receptacle with "GFCI Protected" and "No Equipment Ground" as per NEC 406.4(D)(2)(c), replace the existing 50A breaker for the range with the correct 50A GFCI breaker for your panel, and convert the range to use a 4-wire cord and plug as per the manufacturer's instructions.  This, at least, will provide GFCI protection to the range to address the shock hazard otherwise posed by not having the range's frame grounded.  You'll need to make sure that all the terminal screws are torqued to the labeled specification torque for that device with an inch-pound torque screwdriver, too, since AA-1350 cable is rather finicky about terminations; a good wire brushing of the wires and a light coat of a listed termination (anti-oxidant, no-al-ox) compound before terminating them would not hurt at all, either.