There are some vague mentions around of a ReK Tek beryllium prototype for the '93 Interbike, so maybe this is it?
The TT cable stops look to have been machined to fit a larger diameter of tube, so looks very 'prototype', yet the cast lugs would have been a considerable expense. Did RekTek make a bonded frame already, or are the lugs lifted from Trek or such?
Most likely to be one of the the AlBeMet alloys from Brush-Wellman mentioned in the Ibis quote which Gerard posted, which is allegedy fully weldable. Bonding would still be a good option, given the H&S/COSHH risks of the vapour.
It could be Boralyn (Aluminium/Boron Carbide) alloy. The earliest MMCs (French Aerospace) were based on 2014 aluminium, which isn't weldable, so the bonding may have been essential.
If it were a weldable alloy based MMC then it would surely have been a welded frame? RekTek had plenty of experience welding aluminium frames, so a boron carbide metalmatrix (still 95+% aluminium and fully weldable) tube shouldn't have been any trouble, just like Specialized, Univega, Dean, Clark Kent etc. No need to cast lugs and bond it together. Unless Rek Tek already knew about the problems of the metal fibres migrating away from the weld pool causing weaknesses?...
Material properties:
https://materion.com/products/metal-mat ... s/albemet#
All the best,