Reddit mentions of Straight Blade Devices, Receptacles, Flush Receptacle, Industrial Grade, 3-Pole 4-Wire Grounding, 50A 125/250V, 14-50R, Black, Single Pack

I'm definitely in the "this electrician appears not qualified camp".

My guidance to anyone installing a new circuit for an EVSE is to spend a few more dollars on high-quality parts and future proof the installation a bit on amperage. Remember, the cost for a project like this is mainly labor, not parts (especially with only 3-feet between the receptacle and panel).

With that in mind, I'd order a commercial NEMA 14-50R ($80) receptacle rather than use the cheap ($8) one the electrician will undoubtedly get from the local big box store. I'd also size everything correctly for the full 50 amps, even if your current EVSE and/or BEV charging capacity is smaller than that.

Here are some specifics based on what you shared with us:

• Order these from Amazon and tell the electrician to install them:
  
  • Hubbell Wiring Systems HBL9450A Heavy Duty Specification Receptacle, 50 Amp, 125/250VAC, 3 Pole, 4 Wire, Black
    
    • https://www.amazon.com/gp/offer-listing/B00IPBMUKY
      
  • Hubbell Wiring Systems Receptacle Wall Plate, 2 Gang, 2-31/64" Hole Diameter
    
    • NOTE: As the commercial duty receptacle has a slightly larger diameter you need a matching faceplate. The faceplates from big box store are slightly smaller and won't fit.
      
    • https://www.amazon.com/Hubbell-Wiring-Systems-313292-Receptacle/dp/B002FYJ758
      
• The electrician provides these:
  
  • 50 amp double-pole breaker
    
  • 2-gang deep receptacle box
    
  • Type NM-B 6/3 with ground conductor
    
    • NOTE: Do not let the electrician talk you into using a smaller gauge wiring, such as 8/3. The NEC ampacity table for NM-B cable is pretty clear on this and your guy already seems to not be the sharpest electrician in town.
      
  • Bits and bobs to connect everything up.

Seems like the lesson there is to get a high quality receptacle designed for multiple insertions. The first thread recommends the Hubbell HBL9450A, which is pretty pricey. I find it hard to believe a Leviton one would be fine since that is commonly seen at RV sites.

Also proper installation/wiring is key. All connections must be good so nothing gets hot, and cables should be of the proper thickness for the current drawn (6AWG in this case).

Also a of grease or dust getting onto the plug as that can reduce the contact area and cause heat.

(And amps can equate to heat if wiring is inadequate and thus overloaded.)