byf44 k-block scrubbed blueing or phosphate?

[GunKraut]

I have a k-block byf44 (matching receiver and bolt) with worn blue bolt marked MI. Only stamped eagle, "8" and "S o" at the root of the handle. Receiver, bolt shroud, rear sight sleeve and sight leaf are proofed WR.
AFAIK, phosphate was first observed in the l-block. Any thoughts?

 

[Bob in OHIO]

No phos in k