M 50mm 1.4 Problem

[Thibs]

I got this lens from Ebay and quiclky put it on another purchase (also 
from Ebay) which were Ricoh bellow bellow+tubes+reverse adaptor 49mm (at 
30euros total, not a bad price).

I notice that after staying a couple hours reversed on the bellow, the 
front "thing" (not piece of glass, the part where it is actually written 
Pentax-M and such) is moving and making noise. It does not seem to be 
unscrewed (I can't screw it back).

Any idea on how to fix it? Was so happy to offer a companion to my softy 
55mm 1.8 ;)

Thibouille