What's this for?

[Maker]

It's very thick solder, it came with a box of crap I bought on eBay last year. The thin(er) ones say "HOYT TINNING METAL GUARANTEED VIRGIN SOLDER", one of the wide ones says "FRY D 2521" and the other says "D 1910".

Anyone know what it would be used for?

 

[WorkshopChris]

Lead loading on car body panels?

 

[Tinbasherdan]

Lead loading on car body panels?

Would be my guess too

 

[The_Yellow_Ardvark]

Lead loading on car body panels?

Me to.

 

[gaz1]

via the plumbing side of it

there used for jointing lead to lead or lead to copper pipe jointing

or other things

 

[rtbcomp]

Yep lead loading. I used to use some stuff called "Selflux" it looked like treacle with tin powder in it. You had to stir it before use to get the tin suspended, spread over the area you wanted to tin and attack it with a blowlamp. It bubbled, emitted an evil smell but left a perfectly tinned surface. I then used my own recipe* solder and Fry's flux to build up the area to be repaired.

*I used to save the electrical solder from work (40/60 Pb/Sn) and mix it 50/50 with the lead from car battery plates, this gave 50/50 Pb/Sn with a trace of antimony for hardness.

 

[500e]

Could be body solder 70/30, plumbers is 50/50 the flat bars are more likely one or the other, the thinner triangle or square is usually tinmans solder or as pdg says dip soldering
50/50 has a lot less spread of usable heat range & will sag/run before 70/30 so is not as controllable, lead free is not favorite for body work due to higher melting point so distortion becomes a bigger problem
Just a snippet this is a solder spray gun using acetylene for the heat source stolen from a US site I have one somewhere that used town gas it is expensive