Can anyone help identify this please?

I dont know if anyone can identify what is in the photo. I am demolishing an old wooden greenhouse and took a bracket down holding a shelf . Moulded perfectly into the groove of the metal bracket was this “ structure” which had been made against the wooden shelf. The shelf is undamaged/not chewed/ not holed. It looks like a row of individual cells joined with something like sawdust ( but not from the wood of the shelf) . Each cell is about the size of a piece of sweetcorn.
i have never seen anything like this and wondered if anyone has had experience of it or can identify the critter ?

 

@JWK i can see your thinking but i actually put that shelf up during the pandemic , what you see in the photo has appeared after that time , the pupae , or whatever they are, has deveoped in a tiny gap between the bracket and the wooden shelf

 

Put on gloves, facemask and overalls with a hood and boots. Take one sharp knife and cut them open to see what flies out.

 

Leaf cutter bees????

 

Haggis eggs?

 

@waterbut , thank you , i seem to recall John Hurt, similarly kitted out , trying something similar in the film Alien. I dont think that worked out too well, think i will pass on that.

 

@burnie , i doubt it , i havent grown neeps and tatties this year which i understand is the staple diet of the wild haggis.

 

My neeps are almost ready for pulling. Planted late plugs. My pet haggi are looking forward to them.