the issue at first glance is IRC 731 which says IN GENERAL a partner recognizes gain to the extent that cash (which includes marketable securities at fair market value) exceeds the partner's tax basis in the partnership
however, there is an exception (gain is not recognized) under iRC 731(c)(3)(A)(iii):
such partnership is an investment partnership and such partner is an eligible partner thereof.
(i)Investment partnership (clause i)
The term “investment partnership” means any partnership which has never been engaged in a trade or business and substantially all of the assets (by value) of which have always consisted of—
(I)money,
(II)stock in a corporation,
(III)notes, bonds, debentures, or other evidences of indebtedness,
(IV)interest rate, currency, or equity notional principal contracts,
(V)foreign currencies,
(VI)interests in or derivative financial instruments (including options, forward or futures contracts, short positions, and similar financial instruments) in any asset described in any other subclause of this clause or in any commodity traded on or subject to the rules of a board of trade or commodity exchange,
(VII)other assets specified in regulations prescribed by the Secretary, or
(VIII)any combination of the foregoing.
(ii)Exception for certain activities
A partnership shall not be treated as engaged in a trade or business by reason of—
(I)any activity undertaken as an investor, trader, or dealer in any asset described in clause (i), or
(II)any other activity specified in regulations prescribed by the Secretary.
(iii)Eligible partner
(I)In general
The term “eligible partner” means any partner who, before the date of the distribution, did not contribute to the partnership any property other than assets described in clause (i).
thus i agree with @Anonymous_ you recognize no gain or loss and assume the partnership's tax basis in the securities.
