Pending SBR as Pistol

I want to 100% make sure I’m good to go as I am not looking for any trouble. I have a complete lower that has a pending stamp and I want to use that lower in a pistol configuration before stamp arrives. Here’s the deal:
Lower purchased complete minus a stock, but with buffer tube
Never assembled into rifle
Engraved and tax stamp pending
Has UBR stock attached without bottom half (just tube with cheek rest part)
Lower half of stock will be no where to be found
Want to put 10.3 upper on it until stamp is received
After stamp received put rest of stock on.

Am I missing anything or is this totally legal?

Thanks

im not a lawyer but it has been said here many times. buy a lower without a stock. the 4473 cannot be marked longgun. build it however you want with with either a pistol tube or a buffer tube wihtout a stock. The UBR thing i cannot speak to because I have never seen one upclose but many people just use a normal receiver exension and not put a stock on it. Others just use a pistol tube that cannot accept a stock period. Seems legit but again, Im not attorney.

Thank you for the response. I think I will be good, I just wanted to double check. Here is a pic of what I’m working with.

The picture means nothing. You need to go by how the 4473 was marked. In addition if you do not tell the world, the world shall not know.